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Show 80 ON CLASSIFICATION. Lainellibranchs have the three fundam ental pairs of ganglia of l ad t t 1e on op1tw r a, but they possess no .t race. of the odon tophor. o. Furthermore, they are all provided with. bivalve. external palhal shells, the valves being right and ~eft In relation to the body. N 0 shell of this kind is found In any of. the Odontopho:a. Almost all Lamellibranchs, lastly, have a ~air of _Jmnella.r g11lR on each side of the body, and all are provided With annculato hearts. N 0 doubt the Odontophora and the L am_ellibranchiata ro erly form parts of one anrl the san1e sub-lnngdom, l\IoL~ us~A and the three classes which follow, viz., the Ascidioida, Brachiopoda, and P olyzoa, are usually included in the same subkingdom. But the difficulty of framing a definition which shall include the last-named classes with the L arnellibranchiata and Odontophora iR almost as great as in the parallel case of th Annuloida and Annulosa; while, on the other hand, the Ascidians, Brachiopods, and Polyzoa exhibit many features in coinin on. Thus the nervous system is greatly simplified in all thr e cla ses, consisting, · in the .Ascidioida and Polyzoa, of a single ganglion, sending perhaps a commissural cord r uncl the gullet. In the Brachiopoda the chief ganglia, which appear to be the homologues of the pedal ganglia of the higher mollusks, and are conn cted by a circumoosophageal cord, are combined ·with ace ssory ganglia, but t!1ese do not seem to be identifiable with the pedal or the paneto-splanchuic ganglia. . . .Again, the fact that the heart, when present, Is of a s1m~le tubular, or saccular, character, and is devoid of any separatiOn into auricle and ventricle, constitutes a wid difference between these three classes and the high<'r 1\follusks. On tho other hand, these classes, which may be conveniently denominated MoLL UISCOIDA, resemble one another in the fact (so far as I am aware there is only one exc.eption, Appendic~~la-ria) that the mouth is provided with ciliated t ntac. .u la, disposed in a circle, or in a horse-shoe shape, or fringing lm1g arms ; that it leads into a large, and sometimes an xceedingly large, pharynx; and that in two of the three, at 1 ast, that system of cavities communicating with the exterior, which has been called the "atrial system," is greatly developed. THE MOJ .. LUSCOIDA AND CCELENTERA'rA. 81 I cannot doubt, then, that the Molluscoida form a natural assmnblage; but, until the precise charact rs, if any exist, which unite them with the Mollusca proper can be clearly defined, I am inclined to think it might be better, as in the case of the Annuloida, to recognise them as a separate clivi ion of the Animal Kingdom. The next two classes-the Actinozoa and the Hydrozoa-constitute one of the most natural divisions of the Animal Kingdom -the OmLENTERATA of Frey and Louekart. In all these animals, the substance of the body is differentiated into those histological elements which have been termed cells, and the latter are primarily disposed in two layers, an external and an internal, constituting the " ectoderm " and "endoderm." Among animals which possess this histological structure, the 003lenterata stand alone, in having an alimentary canal, which is open at its inner end and communicates freely, by means of this aperture, with the general cavity of the Lody. In a large proportion of these animals the prehensile organs are hollow tentacles, disposed in a circle around the mouth, and all (unless the Otenophora should prove to be a partial exception to the rule) are provided with very remarkable organs of offence and defence, termed "thread cells " or "nematocysts." These, when well exhibited, as, for example, by the common freshwater polype (Hydra), are oval, elastic sacs, containing a long coiled filament, barbed at its base, and serrated along the edges. When fully developed, the sacs are tensely filled with fluid, and the slightest touch is sufficient to cause the retroversion of the filament, which then projects beyond the sac for a distance, which is not uncommonly equal to many times the length of the latter. These fine :filarnents readily penetrate any delicate animal tissue with which they are brought into contact, and cause great irritation in the human skin when they are of large size. Nor can it be doubted that they exert a similarly noxious influence upon the aquatic animals which are seized by, and serve as prey to, the Actinozoa and Hydrozoa. Characteristic as these organs are of tho Coolenterates, however, it must not be imagined that they are absolutely peculiar to the sub-kingdom; for some nudibranchiate Mollusca, such as Eolis, are urmed with G |
