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Show 6 ON CLASSIFICATION. ntte d I·n an or d ei, ancl arranab oment' the p• urpose of which will appear more fully Ly ca. nd L.v; ' i·n the following table. TABLE OF THE CLASSES OF rriiE ANilHAL KINGDOl\I. The Limits of the Fmw Ouvierian s~~b-J[in,qdO?ns are indicated by the Brackets and Dotted Line. ; Gregarinida. Infusoria. : Rhizopoda (?). l Spongida. i Hydrozoa. : Actinozoa. : Polyzoa. Brachiopoda. Ascidioida. Lamellibranchiata. ' I I I RADIATA. Scolecida (?). Echinodermata. Annelida. ) I Or·ustacea. l A1·achnicla. Awr1 lJiyriapoclct. I Insecta. I J ULATA. Branchiogast~ropoda. Pulmogasteropoda. Pteropoda. Cephalopoda. ~ l\lOLL USCA. I I I Pisces. ) Amphibia. I Reptilia. ) VER'fEDHA'rA. Aves. lJianL?nalia. J GHEGAUINIDA, HIIIZOPODA, S.PONGIDA, AND INFUSORIA. 7 It is not necessary for Iny purpos that the groups which are named on tho preceding ta.b]o should Le absolutely and precisely equivalent one to another; it is sufficient that the snin of then1 iB the whole of the Animal ICingdon1, and that each of then1 embraces one of the principal typos, or plans of mocliflcatiou, of animal form; so that, if we have a precise knowledge of that which constitutes tho typical 'tructuro of each of tho ·e group ', we shall have, so fi:tr, an exhaustive knowledge of the \.nimal lGngdom. I shall oncloavonr, then, to define-or, where definition i~ not yet posl':liblo, to describe a typical example of-the ·o variouH groups. ~ubsoq uontly, I shall take np some of tho e further dassificatory queRtions which arc open to discu. sion; inquiring how far we cau group those classes into larger as,'Onlblages, with definite and constant character · ; and, on the other hand, how far the existing subdivisions of the classes are well Lased or otherwise. But the essential matter, in the first place, is to be quite clear about tho different classes, and to have a distinct knowledge of all the sharply-definable modifications of ani1nal structure which are discernible in the animal kingdom. The first class of which I shall speak is tho group of the GnEGARINIDA. These are ainong the simplest animal forms of which we have any knowledge. ~they are tho inhabitants of tho bodies for tho most part of invertebrate, but also of vertebrate, anin1nls; and they are commollly to be found in abundance iu the alilnentary canal of the comn1on cockroach, and in earthworms. ~rhey aro all microscopic, and any one of them, leaving Ininor 1noclifications aside, may be said to consist of a sac, conlposed of a Inore or less structureless, not very well-defined mmnbrane, containing a soft semi-iluid substance, in the midst, or at one end, of which lies a delicate vesicle; in the centre of the latter is a more solid particle. (Fig. 1, A.) No doubt Inany persons will be struck with the close resemblance of tho structure of this body to that which is possessed by an ovum. You n1ight take the more solid particle to be tho reprosontati ve of the genninal spot, and the voside to bo that of the germinal vesicle ; while tho smni-flnid sarcoclic contents might bo regarded as the yolk, and the outer 1nombrane as the vitelline |
