(1485). The mouth itself is described by Cuvier as being a simple triangular opening, resembling the wound inflicted by a trocar; and in the solitary specimen at his disposal he did not succeed in detecting any dental structures. Eschricht, however, with superior opportunities, was more successful in displaying the oral organs, and found the Clio to possess jaws of very singular conformation, and a tongue covered, as in many other Mollusca, with sharp horny spines.

(1486). One of the jaws removed from the body, and magnified twenty-eight diameters, is represented in the subjoined figure (fig. 278, a) . It consists of a series of sharp horny teeth of unequal length, fixed to the sides of a lateral pedicle in such a manner that their points are all nearly at the same level. The teeth themselves have a golden metallic lustre, and, when examined in the sunshine under water by means of a lens, are especially beautiful objects. The basis to which they are fixed is apparently of a fleshy character, and if smashed by being squeezed between two plates of glass, and then placed under the microscope, appears to be made up of a multitude of regularly-disposed fibres that cross each other in two principal directions.

A, one of the jaws of Clio borealis. B, the tongue, with its recurved spines.

Fig. 278. A, one of the jaws of Clio borealis. B, the tongue, with its recurved spines. C, cylinder enclosing the prehensile suckers.

(1487). The jaws thus constructed are placed on each side of the mouth, contained in two hollow curved cylinders, the walls of which are muscular; and if one of these muscular capsules be snipped by means of a pair of very fine scissors, the strangely-formed jaw, with its teeth, is found lodged within it.

(1488). The manner in which the Clio uses these dental organs is obvious from their anatomical position. The curved muscular cylinders by the contraction of their walls force out the teeth, so that they then project from the mouth, and are ready to seize and drag into the oral orifice whatever food presents itself.

(1489). Once conveyed by the jaws into the interior of the mouth, the prey seized is taken hold of by the tongue; the free extremity and upper surface of which is seen, when highly magnified, to be covered with regular rows of spiny hooklets, all directed backwards, and evidently intended to assist in deglutition (fig. 278, b).

(1490). The structure of the alimentary canal is extremely simple. The oesophagus (fig. 279, i) gradually dilates into a wide stomachal cavity that is surrounded on all sides by the mass of the liver; while the intestine (y), in which the stomach terminates, mounting towards the left side of the neck, ends by an external anal orifice. Two long and slender salivary glands (w) are placed at the sides of the oesophagus, and furnish a secretion that is poured into the mouth. The precise character of the bile-ducts has not been satisfactorily determined in Clio; but in Pneumodermon, another Pteropod very nearly allied to the genus we are describing, the stomach itself, which is enveloped on all sides by the liver, receives the biliary secretion through a multitude of minute pores.

(1491). With respect to the real nature of the respiratory apparatus in Clio, much doubt exists. Cuvier regarded the aliform fins as being subservient to respiration, as well as forming locomotive organs, and observes that the surfaces of these appendages, seen with the microscope, present a network of vessels so regular, so close, and so delicate, that it is not possible to doubt that they are intended to perform the functions of a respiratory apparatus, adding, moreover, that their connexion with the internal vessels and the heart confirms this view of the nature of these membranes.

(1492). Eschricht, on the contrary, denies altogether the existence of any such vascular ramifications as Cuvier describes, asserting that the appearance alluded to is entirely produced by the spreading out of the muscular fibres above-mentioned, and that the only vessels visible in the alar processes are a few arterial branches derived from the aorta.

(1493). We are still, therefore, in ignorance as to the respiratory organs of Clio: the heart, however, is very apparent; it is composed of a single auricle and ventricle, enclosed in a pericardium (fig. 279, m), and gives off at one extremity a large vessel (m), which Cuvier regarded as a pulmonary vein, but which Eschricht has proved to be the aorta, inasmuch as he has traced its branches to the liver and the other internal viscera of the body.

(1494). The nervous system of this mollusk is easily distinguished, not only on account of the large proportionate size of the ganglia, but from the circumstance of the nerves being of a pale-red colour. The ganglia form a ring placed around the oesophagus near the middle of the neck. There are eight large and two smaller ganglionic masses closely aggregated in this situation; and from these sources all the nerves of the body are given off.

(1495). From the large dimensions of the nervous centres, we may be prepared to expect senses of correspondent perfection of structure. We have already mentioned the sensitive tentacula protruded from the hood-like covers that protect the oral apparatus; but, in addition to these, organs of vision are provided, apparently of a very complete character. These eyes are two in number, and are placed on the back of the neck. Each eye has the form of a somewhat bent cylinder, having its two extremities rounded off. The anterior end of the cylinder is the transparent cornea; and when the eye is removed from the body of the animal and examined under the microscope by transmitted light, sundry parts may be detected in its interior - sufficient, indeed, to indicate the existence of a choroid membrane, a vitreous humour, and a distinct lens, occupying the ordinary positions of these parts of the visual apparatus.

(1496). The generative system of Clio resembles in all essential particulars that of the most highly organized Gasteropoda, and, as in them, is composed of a complete set of male organs as well as of ovi-gerous viscera. According to the views which Cuvier was led to entertain from the dissection of a single specimen, he supposed that the ovary (fig. 279, n) gave off a slender oviduct (o) terminating in a thick glandular canal, the testicle (k), which, beginning by a caecal prolongation, and gradually diminishing in diameter until it became attenuated into a slender vas deferens (p), ultimately emptied itself into a small round sac (q) situated in one side of the neck, where it communicated with the exterior. Close to the sac (q) the illustrious French anatomist pointed out another vesicle (r), which he compared to the bladder (spermatheca) of Gasteropod Mollusks. The more complete researches of Professor Eschricht have, however, rendered considerable modifications of the above description requisite, inasmuch as that gentleman has succeeded not only in detecting a testis quite distinct from the ovigerous canal, but also a very complete intromittent apparatus. The testis, in fact, in a fresh specimen is so large as to occupy a great portion of the visceral cavity; and, no doubt, in the individual examined by Cuvier, which had been kept in spirits of wine, it formed a large portion of the mass (fig. 279, i) which he thought to be entirely made up of the liver. The duct from this testis communicates with the receptacle (q); so that the glandular canal (k) must be regarded as a part of the oviduct analogous to what has been called the uterus in the Snail.

Viscera of Clio borealis: mm, the heart, giving off a large vessel.

Fig. 279. Viscera of Clio borealis: mm, the heart, giving off a large vessel; t, oesophagus; v, intestine; w, salivary-glands; n, ovary; o, oviduct; k, testicle; p, its excretory canal; q, bladder-like organ.

(1497). Another important discovery for which science is indebted to the Danish Professor is that the Clio possesses a long and singularly-formed penis (fig. 277, c, h), lodged, when retracted, in the interior of the head of the Pteropod, but which, together with the bladder (g), in which it was contained, can be extruded from the right side of the neck to such an extent that it nearly equals in length the whole body of the little creature.

(1498). The mass formed by the viscera occupies but a small space in the general cavity of the body. The external investment of the visceral sac is a thin semitransparent skin (fig. 279, f) of soft texture; and within this is a second covering (g), thicker than the first, and exhibiting very distinct muscular fibres, principally distributed in a longitudinal direction, so that their action would seem to shorten the animal and make its shape more spherical.

(1499). What fills up the space that intervenes between the muscular tunic and the viscera is as yet undetermined; but Cuvier, in the memoir above referred to, suggests that it may possibly contain air, which, as it should be compressed or allowed to expand, would form a kind of swimming-bladder, and allow the animal to mount to the surface, or sink into the recesses of the sea, with little effort or exertion of muscular power.

(1500). The other genera included in this class agree in their general form, and in the arrangement of their digestive and reproductive organs, with Clio above described, but present a few important modifications in the disposition of their branchiae, and other minor circumstances.

(1501). In Hyalaea the mantle contains a shell composed of two unequal plates, one of which is dorsal, and the other ventral; and the branchiae, which are here distinctly recognizable, form a circle of vascular leaflets enclosed in a cavity of the mantle situated between the divisions of the shell, and so disposed that the water has free admission to them through the two lateral fissures of its testaceous defence.

(1502). In Pneumodermon, again, the branchiae occupy a totally different situation, the branchial leaflets being arranged in semicircular lines upon the posterior extremity of the animal; but such modifications of a general type of structure are of more interest to the zoologist than to the physiological reader, and our space warns us that we have yet to encounter forms of life widely different from any that have hitherto fallen under our notice.