serpentine movements.

The genus Pyrosoma contains animals aggregated in another manner. These are lengthened Ascidians, united to each other so as to compose a long free tube, open at one extremity and closed at the other. This is also an oceanic genus, and is remarkable for being vividly luminous, the light displaying the most gorgeous and varied hues. We shall close this chapter with an extract from Mr Bennet's "Wanderings," in which he records his own observations of these interesting animals. "On the 8th of June, being then in lat. 0° 30' south, and long. 27° 5' west late at night, the mate of the watch came and called me to witness a very unusual appearance in the water, which he, on first seeing it, considered to be breakers. On arriving upon the deck, this was found to be a very broad and extensive sheet of phosphorescence, extending in a direction from east to west, as far as the eye could reach. The luminosity was confined to the range of animals in this shoal, for there was no similar light in any other direction. I immediately cast the towing-net over the stern of the ship, as we approached near the luminous streak, to ascertain the cause of this extraordinary and so limited phenomenon. The ship soon cleaved through the brilliant mass, from which, by the disturbance, strong flashes of light were emitted, and the shoal, judging from the time the vessel took in passing through the mass, may have been a mile in breadth. The passage of the vessel through them increased the light around to a far stronger degree, illuminating the ship. On taking in the towing-net, it was found half filled with Pyrosoma (Atlanticum?), which shone with a beautiful pale-greenish light, and there were also

a few shell-fish in the net at the same time.

After the mass had been passed through, the light was still seen astern, until it became invisible in the distance, and the whole of the ocean then became hidden in darkness as be

fore this took place. and interesting."

The scene was as novel as beautiful

CHAPTER XXVI.

CONCHIFERA AND GASTROPODA (Shell-fish).

SUCH of our readers as are familiar with the fish-market at Southampton have, doubtless, often seen a large sort of shell-fish which are sold under the name of "Old Maids." The fishwoman, perhaps herself an "old maid," will tell you that these humble sisters of hers, these rivals in celibacy, are dug up from the muddy margin of the harbour, into which their burrow descends a foot or more deep; that the hunter for them is aware of their presence by a jet of water spirted from the mouth of the hole as soon as they become aware of his, a fact of which his арproaching footsteps inform them; and that he instantly plunges his narrow spade or fork in an oblique direction, and raises the unlucky bivalve, with a cloud of mud and water, into the air.

Now, if we examine one of these "Old Maids," which naturalists designate as the Sand Gaper (Mya arenaria), we find that we have an animal closely resembling those Ascidia that we lately described. It is enclosed in a leathery wrinkled coat, with the two orifices placed near the end of a rather long tube; the internal anatomy differs little from that of the creatures just named, ex

cept that there is an opening in the side for the protrusion of a moveable fleshy organ called the foot; but externally there is this peculiarity, that, in the substance of the coat, there are formed two valves or convex plates of stony substance (shell), which are united along one side by a hinge, and enclose the soft parts, as the leaves of a book are contained within the covers.

This, then, is a bivalve shell-fish, as it is commonly (though incorrectly) named, and it is the representative of an extensive Class of animals, found all over the world, in fresh and salt waters, and designated CONCHIFERA, or "Shell-bearers." There is, it is true, very great diversity in the details of form and structure that we find in this immense assemblage of animals; but all these variations can be easily traced by insensible gradation to this primal form, and thence to those lower types which we have already described. Sometimes the orifices for receiving and discharging water are prolonged into two distinct tubes with fringed extremities, as in that lovely and delicate shell that inhabits our sandy beaches, called, from its diverging rays of pink and yellow, the Setting Sun (Psammobia vespertina), where the two tubes, when fully extended, are twice the length of the shell. At other times we find the tubes again reduced to simple openings, and one of these forming a mere slit, scarcely to be distinguished from the common opening of the coat or mantle, as in the Mussels (Mytilida). This separation of the mantle, again, occurs in various degrees, from its condition as a mere orifice for the protrusion of the foot, to that of the Oysters (Ostreada), where it is open all round, a fleshy counterpart of the shelly valves, bordered by a

short but close fringe. In the beautiful Pectens, "the butterflies of the MOLLUSCA," the mantle is still further modified, for it is furnished with four rows of long moveable contractile tentacles, and with two rows of eyes that sparkle and glow like the most brilliant gems.

Another and a parallel modification takes place in the breathing organs, which, instead of being a

Pecten.

closed sac, as we saw it in the Ascidia, become thin flat leaves, much like the folds of the mantle or the shell-valves, which are placed a pair on each side. Their structure is no less modified than their form, for instead of oval ciliated cells on the internal surface, each of the four leaves (in the Pecten, for instance) consists of a vast number of straight, slender, transparent filaments, evidently tubular, arranged side by side, so that 1500 of them would be contained within the length of an inch. Strictly, however, these are but one filament, excessively long, bent upon itself again and again, at both the free and the attached end of the gill-leaf, throughout its whole extent. This repeated filament is armed on each of two opposite sides with a line of vibrating cilia, the two lines moving in contrary directions, exactly as on the tentacles of the POLYZOA, which are the breathing organs there; by this action a current of water is made continually to flow up and down each of these delicate filaments; so that the blood which circulates in their interior (for they are, doubtless, blood-vessels) is continually exposed, throughout this its long and tortuous course, to the action of oxygen.