forms, the former being as much as possible elongated longitudinally, the latter attaining the extreme of concentration about a centre. At the same time there is a reduction of parts to a minimum, as shown in the absence of a second adductor, and of any foot in the adult state. The differences between these forms are, however, decidedly less than those which may be observed between the extreme forms among the Cephalopoda or Gasteropoda.

The Pteropoda and Pulmonata.-The Lamellibranchs are, as we have said, curiously exceptional in presenting the general features of the Mollusca proper, without that singular buccal apparatus which we meet with in all other members of the subdivision, whether neural or hæmal, and whose peculiar nature is described below. Again, they are exceptional in the vast development and symmetrical longitudinal division of their mantle, and in the corresponding division of their pallial shell into two pieces or valves-characters we shall not meet with again in any modification of the Common Plan.

In the Pteropoda and Pulmonata the mantle is never developed into such lateral lobes, and the shell to which it gives rise never consists of two pieces, but is constituted by a single mass, which either has the form of a flat plate or presents some modification of a cone. Again, the foot (or some part of it) is always well developed, presenting no obvious distinction into regions in the Pulmonata; but in the Pleropoda often exhibiting a well-marked meso- and metapodium, and always presenting a characteristically large epipodium-an organ which in these Molluscs constitutes the so-called 'wings,' from which their name is derived.

Fig. 5.

founding together of parts so essentially different has taken place, we shall find, in the Nudibranchiata and in the Heteropoda.

The Cephalopoda.-In the Pteropod forms, Pneumodermon and Clio, a hood, giving off long processes covered with suckers from its inner surface, surrounds the oral aperture, and there is every reason to believe corresponds with the propodium, whose lateral halves have united over the mouth. If the like process were to take place in a Criseis, but to a greater extent, so that the mouth were thrust back between the halves of the mesopodium, and the propodium and mesopodium formed one continuous tentaculigerous sheath around the oral aperture; and if at the same time the two halves of the epipodium united posteriorly into a funnel-shaped tube, the Criseis, so far as its external orgánisation goes, would no longer be a Pteropod, but would have become a Cephalopod. In fact, the Cephalopod may be derived from the Archetype by supposing these modifications. The mantle is always well developed, and its cavity incloses one or two pair of gills. The two halves of the epipodium are united behind into what is called the funnel, a peculiar apparatus, of great importance in the economy of many Cephalopods; and in the majority of the group the sides of the foot, having united in front of, and forming a complete sheath for, the head, are produced into eight or ten processes, the so-called arms, on which are set the acetabula, or suckers. Fig. 6.

There is usually a well-developed mantle in the Pteropoda and Pulmonata, and its walls act as a branchial surface without being produced into true gills-(Hyalaa?)—the sea-water in the marine Pteropoda and the air in the terrestrial and aquatic Pulmonata being inspired and expired into its cavity.

In the Pteropoda in general, the aperture of the pallial cavity and that of the anus, are situated upon the posterior surface of the body, in accordance with the neural flexure of the intestine. The anal aperture however is usually thrust to one side of this surface, and, in Limacina and Spirialis, this lateral thrust has taken place to such an extent, that not only the anal aperture, but that of the mantle cavity, is thrown up completely on to the dorsal surface. This laterodorsal, or dorsal position of the anal and respiratory apertures, is as regular in the Pulmonata as it is exceptional in the Pteropoda.

In the Pteropoda and Pulmonata some most important modifications of form are produced by the greater or less development of the mesosoma on the one hand and of the mantle on the other. The predominance of the latter is to be observed in such forms as Criseis, Cleodora, Hyalæa, and Helix; while the former may be seen in Pneumodermon and in Limax. In the latter the mantle is very small, and in the former it is almost if not entirely absent; what is ordinarily considered as the mantle in this mollusc being in fact nothing more than the mesosoma. The like con

ep

Vertical Section of Loligo media.

m, shell; a, cerebral ganglia; y, pedal ganglia; s, parieto-splanchnic a, oral aperture; b, anal aperture, or extremity of the intestine ; m, mantle; ganglia; ep, funnel.

Beyond this peculiar arrangement and development of the external organs, we are not aware that any characters exist by which the Cephalopoda, as a class, can be distinguished from the other Mollusca. Among themselves they present a remarkable harmony, differing chiefly in the number of their branchiæ, in the internal or external position of their

shell, and in the nature of the appendages into which the edges of the foot are modified-characters which do not attain to ordinal importance in other divisions of the Mollusca.

Having thus glanced at all the leading modifications of the Neural Plan, we may next turn to the Hæmal Plan, commencing with its Molluscoid modification constituted by the Ascidioida alone.

The Ascidioida.-As a Molluscoid group, the Ascidians are characterised, in the first place, by the rudimentary condition of their whole neural region, and by the reduction of their nervous system to a single infra-oesophageal ganglion. Besides these, however, their organisation presents certain characters which appear at first sight very remote from such a Common Plan as has been described, and hardly deducible from it. An Ascidian, in fact, is usually fixed by one extremity of its body, and presents at the other two apertures. One of these leads into a wide cavity, whose entrance is fringed with a circlet of tentacles, and whose walls (except along the middle line anteriorly and posteriorly) are perforated by innumerable ciliated apertures, and often thrown into folds, by which their surface is greatly increased. At the bottom of this cavity-the branchial sac-a second wide aperture leads into the alimentary canal, which invariably presents a hæmal flexure, and then almost always bends backwards neurally to terminate in a second wide cavity. This, the atrium, whose more external portion is usually termed the cloaca, opens externally by the second or cloacal aperture, and extends along each side of the branchial sac up to its median line of attachment-communicating freely with its cavity by means of the small ciliated apertures which have been mentioned. The single ganglion lies between the oral and cloacal apertures.

aperture remaining on the hæmal side, it is bent round, by the same process as in Spirialis and Limacina, but in the inverse direction. Suppose with all this that a mantle has been developed, and that its free margin remaining small and narrow, has followed the anus to the neural side, while its cavity has extended up on each side of the pharynx to the middle line of the hæmal surface of the latter, carrying to a great extent a process of which the outline may be seen in Cymbulia, and giving rise to the atrium;-imagine also that the sac thus constituted externally by the inner surface of the mantle (third tunic), and internally by the pharynx, becomes perforated by minute apertures-and the result would be an Ascidian.

Such is the manner in which the Ascidian type is derivable from the Common Plan. Of this type the group presents three subordinate modifications. The first is that presented by the extraordinary and instructive genus Appendicularia (fig. 7, 7), which in a manner represents permanently the larval state of the more perfect members of the groupswimming by means of a long rapidly-vibrating tail, like that of a tadpole. In Appendicularia there is no cloacal aperture or atrium. The mouth opens into a wide pharynx representing the branchial sac of other Ascidians; from this a gullet leads into the stomach. The narrower intestine passes from the stomach, forwards and to the hæmal surface, where it terminates without bending downwards, and without being surrounded by any special cavity. Appendicularia therefore might be said to be a form in which the process of modification of the Molluscan Archetype into the Ascidian Type is arrested half way.

In all other Ascidians this process is complete, and there is a distinct cloacal aperture and atrium; but these forms again may be arranged under two great sub-typical modifications, according to the development of the branchial sac relatively to that of the post-abdomen. In such forms as Cynthia, Boltenia, Perophora, Botryllus, the branchial sac attains so great a proportional size as to occupy the whole, or nearly the whole, length of the body, the intestine lying on one side of it: these might therefore be well denominated Ascidia Branchiales, Branchial Ascidians. On the other hand, in Clavelina, Aplidium, Polyclinum, Salpa, the alimentary canal lies completely behind the branchial sac, which is proportionally small, and these might therefore be termed Ascidia Intestinales, Intestinal Ascidians. A very complete mutual representation will be found to obtain between the members of these two groups.

Hamal Mollusca.-In passing from the Hamal Molluscoida to the Hamal Mollusca, we find the same new features presenting themselves as in the Neural Division, the transition being even more abrupt, from the absence of any representative of the Lamellibranchiata. In all these Mollusca, in fact, there is a more or less well developed foot; a distinct head, with its organs of sense and buccal armature; and three pairs of ganglia-cerebral, pedal, and parietosplanchnic.

The modification of the Common Plan is carried to a less extent in this than in the Neural Division, the chief varieties of its forms depending on the changes in the shape of the shell with which the majority are provided; on the greater or less development of the different regions of the foot; but most of all in the relative proportions of the mesosoma and mantle.

If we divide the Hamal Mollusca into two great groupsthe one consisting of the Heteropoda, Scutibranchiata, Tubulibranchiata, Pectinibranchiata, and Cyclobranchiata, families, which are most intimately allied, and which are connected as a group by the dioecious arrangement of their reproductive organs; and the other of the Nudibranchiata, Inferobranchiata, and Tectibranchiata, families in like manner united, among other characters, by their common herma

phora. 5, Clavelina. 6, Salpa. 7, Appendicularia.

a, oral aperture; b, anal aperture, or the extremity of the intestine; d, cloacal aperture and atrium; f, branchial sac; g, hypo-pharyngeal band; m', test; q, genitalia; y, pedal ganglia.

Now, in what manner is this form derivable from the Archetype? It is to be remarked, in the first place, that the pharynx, large in the Polyzoa, becomes comparatively enormous in the Ascidians; while the tentacles, which were very large in the Polyzoa, are in the Ascidians comparatively small. Next, with the development of a post-abdomen, the intestine acquires a hæmal flexure; but instead of the anal

tremes of form-the one resulting from the great development of the palliai region, the other from that of the mesosoma. In the Dicecious Division, Dentalium, Vermetus, Atlanta, and the ordinary Pectinibranchiata may be regarded as examples of the former case; and in the Monoecious Division the Inferobranchiata and Tectibranchiata; while the mantle becomes rudimentary or absent altogether in the Dioecious Firoloides, in the Monoecious Phyllirhoë, and the Nudibranchiata in general, where the region from which the so-called branchial processes arise, and which is commonly called the mantle, is not the homologue of the mantle of Atlanta for example, but of its mesosoma, which

Fig. 8.

pp

a, oral aperture; b, anal aperture, or the extremity of the intestine; mt, mantle; ms, mesopodium; pp, propodium; e, ventricle.

The foot in the Monoecious Hamal Mollusca rarely presents any special development of its different regions, except that in certain forms-namely, Aplysia and Gasteropteron-the epipodium is as well marked as in the Pteropoda, and serves the same end in locomotion. This is well known in Gasteropteron, and we have seen a tropical Aplysia 'fly' through the water in precisely the same way as a Pteropod would do. These epipodial lobes have been frequently called mantle, although the true mantle is a most distinct and obvious structure.

In the ordinary Pectinibranchiata, on the other hand, the foot may not be differentiated into its subdivisions at all, the metapodium being marked only by the position of the operculum, when this exists, as in Buccinum. In other cases, as in Oliva and Sigaretus, a deep cleft marks off a very distinct propodium from the conjoined mesopodium and metapodium; in others, as in Pteroceras, the metapodium is as specialised as in Atlanta; while again, in such forms as Natica, the three constituent parts are distinguishable-the propodium constituting the hood in front of the head; the mesopodium the creeping disc; and the metapodium the operculigerous lobe. (Fig. 9.-2 and 3.)

Having thus passed in review those modes of arrangement of the various organs of the Mollusca which constitute the Common Plan of the group and the subordinate plans of its leading subdivisions, we have next to consider the peculiarities presented by these organs themselves, or, in other words, those more striking features in which the organs of the Mollusca differ from those of the Vertebrata, Annulosa, and Radiata. The most important organs, in this point of view, are those of-1, the Alimentary; 2, the Circulatory; 3, the Respiratory; 4, the Renal; and 5, the Nervous System.*

1. The Alimentary Organs, in certain Mollusca, present two kinds of apparatus which are met with in no other division of the Animal Kingdom. The first of these is that peculiar manducatory instrument usually called the 'tongue,' which is possessed by all the Mollusca proper, except the Lamellibranchiata; and for the first description of whose true structure and mode of action we are, we believe, indebted to Mr. Thompson (see article Tongue,' in the 'Cyclopædia of Anatomy and Physiology '), although the organ itself had been more or less an object of attention ever since the time of Cuvier.

a, oral aperture; mt, metapodium; ms, mesopodium; pp, propodium. In the Diœcious group the epipodium is never well developed, presenting itself at most under the form of little lobes and processes-at least it would seem probable that the neck-lappets and head-lappets of the Trochida are rudiments of the epipodium. On the other hand, it is in this group that the propodium, mesopodium, and metapodium attain their most complete and distinct form; as in Atlanta, where the propodium constitutes the anterior flattened fin, the mesopodium the rounded sucking disc, and the metapodium extends backwards, as the tail-like lobe which carries the operculum. In Firoloides we find that the mesopodium

Tongue of Patella.

1. a a, the cartilaginous plates which constitute the pulley over which the elastic plate 2, b, supporting the series of teeth c, plays; dande are the anterior and posterior insertions of the intrinsic muscles of the tongue. 3 is a side view, and 4 a view from above, of the entire apparatus.

The tongue is essentially composed of a cartilaginous mass, with a pulley-shaped upper and anterior surface, which projects from the bottom of the oral cavity. An elastic plate plays over the pulley, and is attached at each end to muscles which arise from the upper and lower surfaces of the cartilaginous mass. Along the middle line of this elastic plate successive transverse series of strong recurved teeth are set -new ones being continually formed behind as the old are worn away-in a sort of persistent dental sac.

When the tongue is brought into play it is protruded by appropriate muscles from the cavity of the mouth, and its extremity is firmly applied against the body to be rasped. The superior and inferior sets of muscles, which are inserted into the corresponding ends of the elastic plate, now contract alternately, and the resulting action is precisely that of a circular saw. It is by means of this apparatus that the Carnivorous Mollusca bore through the shells of the animals upon which they prey; and perforated shells, which have been thus emptied, abound on every coast.

far as we at present know, to the Mollusca, is what is termed The other appendage of the alimentary canal peculiar, so the Crystalline Style, a transparent, usually elongated body, which projects by one end into the stomach, and is lodged for the rest of its extent in a sac formed by a diverticulum of

systems, whose peculiarities, however, are less exclusively Molluscan. Our limits preclude the consideration of the tegumentary and genital

unknown.

Among the alimentary appendages, the Liver in one group, the Ascidians, departs sufficiently from the ordinary plan to deserve particular notice. In these animals (fig. 7, No. 4, 4) it always consists of a series of narrower or wider anastomosing tubules, commencing in cæca upon the outer surface of the intestine, which they envelop in a close network, and terminating by a narrow duct, in the stomach. In the Botryllida the hepatic tubules are remarkably wide.

2. The nature of the Circulatory System in the Mollusca is at present in some respects a vexed question, more especially as regards the important point whether they possess a true closed system of vessels or not. Without entering into any discussion of the various arguments used on both sides of a dispute which is in some respects verbal, we may be permitted shortly to state our own conclusions on the subject. In the Polyzoa there are no special circulating organs, if we except the cilia with which the perivisceral cavity is often lined, and which keep up a continual current in the perivisceral fluid; nor do we imagine that any one will insist that in them the perivisceral cavity is not a sinus, but has a truly venous lining membrane.

In the Ascidians there is a heart, but it is a simple muscular sac, open at each end, and possessing the extraordinary power of reversing the direction of its contractions, and thus circulating its blood first in one way and then in the opposite. The blood thus poured out is driven through channels in which assuredly no separate lining membrane is demonstrable. Indeed it is difficult to comprehend how any one with a living Ascidian under his microscope can question that here, at any rate, the circulation takes place through lacunæ, and not through vessels with distinct walls.

In the Brachiopoda a very remarkable vascular system has been said to exist, consisting of two hearts (in Rhynchonella of four), each composed of an auricle and a ventricle; the former being in free communication with the perivisceral venous sinuses (perivisceral cavity, nobis), while the latter ends in an aorta, whose branches undergo a regular distribution. Such is the circulatory system in the Brachiopoda according to Professor Owen; but our own inquiries have tended to strengthen very greatly the doubts first raised by Mr. Hancock as to the true nature of this so-called circulatory system. It fact these inquiries lead us to doubt whether the so-called 'hearts' of the Brachiopoda have anything at all to do with the circulating system; inasmuch as, in the first place, we are pretty confident that no arteries' are given off from the apices of the ventricles,' as has been said, and think it more than probable that they open externally. Secondly, there is no evidence at present, either indirectly from structure or directly from observation during life, that the so-called 'hearts' of any Brachiopod are contractile. Thirdly, the multiplication of these hearts to four in Rhynchonella seems not a little to militate against their cardiac nature.

We may fairly conclude then that, for the present, the nature of the circulatory system in the Brachiopoda must be regarded as an open question.

Mollusca Proper.-The doctrine first advocated by M. Milne-Edwards that in these Molluscs the circulating system is always more or less incomplete, has met with a wide acceptance, but also with no small opposition. So far as the minute transparent Molluscs, which can be submitted to direct microscopical observation during life, are concerned, we do not understand how the truth of M. Milne-Edwards's doctrine can be questioned. If the term 'venous lining 'is to have any meaning but a non-natural one, assuredly it cannot be said with truth that anything of the kind exists in the sinuses of Firoloides, or of Atlanta, or in those of the Pteropoda.

In the larger Mollusca, on the other hand, much depends on the verbal question-what is the definition of a vein,' or venous membrane ?' If a lamina of connective tissue separable from the surrounding parts be a venous wall, then doubtless the venous blood-channels of many Lamellibranchs and Gasteropods, and perhaps of all Cephalopods, are veins. If on the other hand a greater histological differentiation corresponding to that which exists in the Vertebrata be required to constitute a vein, evidence of the existence of any

thing of the kind in the greater proportion of the venous blood-channels of these creatures is at present wanting.

In the

As regards the grosser structure of the circulatory apparatus in the Mollusca proper, it may be observed that, in the Lamellibranchiata there is either a single auricle and a single ventricle (Ostræa), a single ventricle and a double auricle (most Lamellibranchs), or two auricles and two ventricles (Arca). In all other Mollusca, except the Cephalopoda, there is a single auricle and a single ventricle. Cephalopoda the heart is essentially similar to that of the Lamellibranchs, inasmuch as it consists (in the Dibranchiata) of a single ventricle and of two contractile, so-called Branchio-Cardiac Veins,' which represent the two auricles of the Lamellibranchs. The circulation in these creatures is assisted (at least in Loligo media, in which we lately had opportunities of convincing ourselves of the fact), not only by the regular contraction of the so-called branchial hearts,' which are dilatations of the afferent branchial veins, but by that of the gills themselves.

The nature of the so-called Pericardium in the Mollusca has been much misunderstood. It is most important to recollect that in no case is there evidence of its being a closed serous sac, comparable to the pericardium of the higher animals. On the contrary, wherever it has been examined with sufficient care (Lamellibranchiata, Pteropoda, Heteropoda, Nudibranchiata, and Cephalopoda), it has been found to be a blood-sinus, which in some cases (Pteropoda, Cephalopoda (?), Lamellibranchiata (?), and Heteropoda) communicates with the exterior by the mediation of the renal organ.

3. The Respiratory Function is performed by modifications of several distinct parts in the Mollusca.-1. By the general surface of the pallial cavity, which may be more or less adaptively modified: this kind of respiratory organ is to be found in the Brachiopoda, Pteropoda, and Pulmonata. 2. By specially modified parts of the walls of the pallial cavity into true gills: the whole tendency of the modification of form which these gills undergo is to increase their surface, and this end, generally speaking, is effected in one of three ways-a. By the development of simple processes, as in Patella or Atlanta. b. The simple processes become ramified, so that the gill eventually consists of a stem with lateral branches, and these again may be subdivided into smaller and smaller branchlets-Pectinibranchiata and Cephalopoda. c. In the Lamellibranchiata each gill essentially consists of a stem with lateral undivided branches, and in such forms as Trigonia and Nucula (fig. 4, No. 3, t); the branchia have precisely this structure. In Nucula the lateral branches are comparatively short, but in Trigonia they are much longer. In Pecten they turn up at their free ends upon themselves and form a close loop, so that the free end takes a position near the fixed extremity; at the same time lateral processes are given off from the branches which unite and connect them together by a very loose and open vascular network. Each gill has thus become a flattened pouch, completely open, both laterally and superiorly; the sides of the pouches are very open, and are constituted superficially by the parallel produced and reflected portions of the gillbranches, and more deeply by the very loose network formed by the anastomosing lateral processes. Now, if we suppose that the reflected portion of the outer gill-pouch adheres to the mantle, while the reflected portion of the inner gillpouch remains free on each side of the foot, but adheres to its fellow behind the foot, thus forming a complete partition across the pallial cavity, the deep vascular network becoming very close, and giving off vertical septa, by which the pouch becomes divided into successive antero-posterior chambers; then the result will be such a gill as we meet with in the Oyster, the Unio, and the great majority of Lamellibranchiata. The minute structure of these branchiæ strikingly resembles that of the branchial sac of the Ascidians, as has been long since pointed out by Siebold and others, and has given rise to the prevalent idea that the two organs are homologous. Structural resemblance, however, is in itself no true basis for the establishment of homologies, and here there are abundant means of demonstrating the resemblance to be simply analogical. 3. The 'branchia' of the Nudibranchiata again doubtless subserve respiration, but they are developed from the mesosoma, and contain the gastro-hepatic processes of the alimentary canal-features by which they are essentially distinguished from true gills. 4. The branchial sac of the Ascidians is, as we have shown, a modification of their pharyngial sac, resembling