THE BRITISH STAR-FISHES.

BY GEORGE S. BRADY, M.R.C.S.

(With a Tinted Plate.)

Ir is not at all the object of the present paper to give any general account of the natural history of the Star-fishes, but simply to notice a few points in their structure and economy which may be interesting to the general reader, and especially to the microscopist. And to be easily intelligible we must first briefly describe the external structure and configuration of the creatures to which we refer.

The sub-kingdom Radiata of the older naturalists is now, in accordance with the results of recent investigation, broken up and reconstituted. The fundamental idea of a radiated structure was, however, so far as mere external appearance went, correct enough, and no better type of such an organization could be desired than that supplied by the common Star-fishes. Each Star-fish consists of a central disk, from which are given off symmetrically, like the spokes of a wheel, the arms, or to speak technically, the rays. These rays are mostly five in number, but in some species seven, twelve, or thirteen may be observed. The common Sun-star (Solaster papposa), one of the largest and handsomest, as well as one of the most common species, has usually thirteen rays; while the still commoner Urasters, found everywhere between tide-marks, are examples of the five-rayed form. The number five is, so to speak, the key-note of the class Echinodermata, and may be found in all parts of their external organization, continually recurring either singly or in multiples.

The under surface of a Star-fish exhibits a central orifice, which acts both as mouth and anus; and diverging from it on the under surface of each ray is a longitudinal space or "avenue," furnished on each side (except in the Ophiurada, Sand-stars and Brittle-stars, where moveable spines take the place of suckers, and in which the "avenues" are absent) with innumerable retractile suckers or feet, which serve as organs of locomotion. The Star-fishes are divided according to their mode of locomotion into Spinigrades, moving by means of spines-Cirrhigrades, by suckers-and Pinnigrade, by fins or pinnæ. Of the last-named division we have only one British genus, Comatula, concerning which some remarks will be found toward the end of this paper. At the very extremity of each ray is an organ like an eye, having spinous appendages, which are termed the eyelids. It is doubtful, however, whether these parts have really any visual endowment; no proof of their possessing the faculty of sight has ever been advanced, and from

what we know of the nature of this sense generally in the lowest forms of animal life we should be disposed to consider that the organs in question must serve some other as yet unknown purpose. Other animals not far from the Star-fishes in general organization, though possessing, so far as we know, no special visual apparatus, are yet extremely sensitive to light, and this sensitiveness seems to consist of a nervous irritability shared by every part of the body, higher in degree, but possibly of a nature somewhat analogous to that of similar manifestations observable in plants.

The upper or dorsal surface of the Star-fish presents appearances differing very considerably in different genera. It may be perfectly smooth; it may be rough with tubercles or bristling with spines, and it is the various forms of these appendages to which we wish at present specially to devote our attention. The genera Ophiura and Ophiocoma (Sand-stars and Brittle-stars) may be easily recognized by the great length and tenuity of their rays, and their excessive fragility. The whole surface, both of disk and rays, is covered by scales which are so closely appressed as to give an almost perfectly smooth surface. These scales are arranged in definite and often in very beautiful patterns, and in some species the primary scales are edged or encircled by series of circular bosses or tubercles, giving an exquisitely "rosulated" aspect to the disk and rays. An example of this is given in Fig. 1 of the tinted plate, which is a representation of the disk and bases of the rays of Ophiocoma bellis. Though the surface of the disk is in mature individuals of these genera mostly smooth, many of them are when young closely beset with spines, and in some cases these are persistent even in mature age. The disk of Ophiocoma Goodsiri is not unfrequently thus clothed. Its spines are represented at a in the tinted plate; b of the same plate illustrates the spines, which cover the disk of young Ophiocoma rosula. The rays are in these genera constantly provided with numerous spines which serve as locomotive organs, differing, however, very widely in length and strength. Ophiocoma filiformis has its spines frequently tipped with curious anchor-shaped processes (fig. e), which are supposed to facilitate the motion of the creature over the muddy bottoms which it frequents. In Ophiura they are very short, and not apparent without careful inspection; while in Ophiocoma they are so long as to give quite a bristly, spinous appearance to the animal, being sometimes, in fact, very much longer than the breadth of the rays. The gradual development of the spines of O. rosula has been carefully studied by Mr. Hodge.* In their early stages they are armed with hooks at the extre*Transactions of Tyneside Naturalists Field Club, vol. v., p. 41.

mities, but by successive deposits of calcareous matter at the base of the spine, the hooks are constantly pushed forwards and finally obliterated. They are, however, occasionally observed in mature specimens, and in cases where a rejected limb is in process of reconstruction. Fig. c in the woodcut is copied from Mr. Hodge's illustration of the "hooks" of young Ophiocoma rosula, and Fig. d represents one of the spines of an adult specimen. The common "Cross-fishes" (Uraster) of our coast have a reticulated surface, set with lines of short, but sharp spines these differ in length and strength according to species. In Uraster glacialis they are very strong, and arranged in well marked ridges. In this genus are constantly noticed the peculiar appendages termed Pedicellaria, of which we shall have more to say further on. Cribella has a non-reticulated surface, but in lieu of the network and spines is covered with small tubercles mostly irregularly arranged. About these there is not much to interest the microscopist.

Among the largest and handsomest of British star-fishes is Goniaster equestris, the Sea-pincushion, as it is called by our northern fishermen. The appellation is not inapt, for the large fleshy mass of the animal is covered thickly with round bosses or tubercles of the size of a large pin's head. Each of these is encircled at the base by a series of smaller tubercles, generally from twenty to thirty in number. Interspersed among the tubercles over the whole upper surface, but still more numerously beneath, are certain pincer-like organs, composed each of two flattened lips or valves, which fit together accurately, and which during the life of the animal may be seen in constant motion, opening and shutting, so at least say the fishermen. I have never myself seen the creature alive. I do not, however, doubt the fact. Dr. Johnston states that the radiating circles of spines of Lidia (described further on) have the power of voluntarily opening and closing in a similar manner; and he also noticed the same phenomenon in the present species. These pincers are likewise surrounded by circles of small tubercles. The pentangular disk and rays are bordered by a double row of flattened plates, each of which bears three or four large tubercles, and is encircled by a series of minute ones in the same way as the simple tubercle of the disk. The elaborate ornamentation and great size of this species make it a very desirable addition to the cabinet. It is often brought in in considerable numbers by the trawlers on our north-eastern coast, and specimens not unfrequently measure ten or eleven inches in diameter.

A striking contrast to this large and fleshy species is Palmipes membranaceus, the "Bird's-foot Sea-star," which is almost as thin as parchment, and might, as Professor Forbes

says, be readily mistaken for the torn-off skin of some bulkier species. Its surface is covered with slightly elevated tubercles, which bear very closely-set fasciculi of short and sharp spines. It is one of the rarest of our native forms, being altogether an inhabitant of deep water, and is mostly found very much denuded of its spines.

Asterias aurantiaca and Luidia fragilissima* present a surface structure very different from any of the species previously noticed, their tuberculated epidermis being so closely set with upright spines, as to be almost wholly invisible. These spines are arranged in a radiated or rosette-shaped manner, and have a roughened surface. A portion of the ray of Luidia forms a microscopic object of exquisite beauty: it is represented in Fig. 2 of the tinted plate. The excessive fragility which characterizes many star-fishes is in no instance better shown than in Luidia. Forbes gives the following graphic account of the capture of one of them, probably off the coast of the Isle of Man :-"It is the wonderful power which the Luidia possesses, not merely of casting away its arms entire, but of breaking them voluntarily into little pieces with great rapidity, which approximates it to the Ophiure. This faculty renders the preservation of a perfect specimen a very difficult matter. The first time I ever took one of these creatures, I succeeded in getting it into the boat entire. Never having seen one before, and quite unconscious of its suicidal powers, I spread it out on a rowing bench, the better to admire its form and colours. On attempting to remove it for preservation, to my horror and disappointment I found only an assemblage of rejected members. My conservative endeavours were all neutralized by its destructive exertions, and it is now badly represented in my cabinet by an armless disk and a diskless arm. Next time I went to dredge on the same spot, determined not to be cheated out of a specimen in such a way a second time, I brought with me a bucket of cold fresh water, to which article star-fishes have a great antipathy. As I expected, a Luidia came up in the dredge, a most gorgeous specimen. As it does not generally break up before it is raised above the surface of the sea, cautiously and anxiously I sunk my bucket to a level with the dredge's mouth, and proceeded in the most gentle manner to introduce Luidia to the purer element. Whether the cold air was too much for him, or the sight of the bucket too terrific, I know not, but in a moment he proceeded to dissolve his corporation, and at every mesh of the dredge his fragments were seen escaping. In

*Luidia fragilissima, Forbes, is now sub-divided into two species, L. Sarsii and L. Savignii, the one having five and the other seven rays. The two species, however, present no structural differences.

despair I grasped at the largest, and brought up the extremity of an arm with its terminating eye, the spinous eyelid of which opened and closed with something exceedingly like a wink of derision." It may be added, that even had the Professor succeeded in "introducing Luidia to the purer element," it is very probable that his expectation of procuring a perfect cabinet specimen would not have been realized, for fresh water, though it mostly seems to paralyze and kill star-fishes instantaneously, thus preventing their breaking up, does not act so favourably on Luidia. The creature may be put at once into cold fresh water, and will lie there for some time breaking itself to pieces. The only way in which (so far as I know) this can be avoided is to press the disk of the animal forcibly down with the hand until it is dead. In this way its destructive contractions may be prevented.

The cirrhigrade star-fishes are furnished with certain curious appendages, the use of which is at present very imperfectly understood. These are the "pedicellaria" and "madreporiform tubercle." The latter is a rounded, cushion-like eminence of considerable size, situated on the disk, mostly very much out of the centre. It is irregularly fissured in a radiate manner, and is not at all unlike the animal from which it derives its name. It is found to communicate beneath with a short canal, which is connected with the vascular system of the star-fish. Various conjectures have been made as to the use of the madreporiform tubercle. It has been supposed to act either as a secreting organ for the calcareous elements of the animal, or to filter out the impurities of the circulating fluid. Professor Forbes looks upon it as being merely the analogue of the stalk which exists in the young condition of the crinoid star-fishes: if this be really the case it must be considered as a rudimentary organ without any specific function. The pedicellaria are pincerlike organs which are irregularly scattered over the surface of the animal, and which have distinct characters in the different species. They were for long supposed to be parasitic creatures, but are now generally admitted to be true epidermic appendages. They are in constant active motion during the life of the star-fish, and grasp firmly anything which is brought between their blades.* Their nearest analogues are the birds'head processes which occur in certain zoophytes, and which Mr. Gosse believes to serve the purpose of seizing organic matters, and by holding them in their grasp to provide food for the animal in an indirect manner; for the decomposition of the captured morsel in time attracts round it clouds of infusoriæ, which are swept into the stomach of the polyp by its vigorous ciliary action. It is needless to say, that although

* One of the pedicellariæ of Uraster rubens is shown at ƒ, in the woodcut.