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Show 78 ON CLASSIFICATION. as a N emertid-provided with the charactori tic probosciR, and the other organs of that group of Turbellaria. Fig. 37. Fig. 37.-P ilidiurn gyrans (after Lcuckart and Pagenstecher). A. Young Pilidiurn; a, alimentary canal; b, rud iment of the Nemcrtid. B. Pilidiurn with a more advanced N"cmertitl. C. Newly-freed Nemertid. Many Trematoda., and all 1.1cenioid Scolecida, again, present an essentially similar IJrocess of internal gemn1ation, in viTtuo of which either a separate offspring arises, or an adult is developed within an embryonic form; but in these cases tho appropriation of the intestine of the primary by that of tho secondary form, which renders the ordinary development of the Echinoderm so striking, does not occur. In discussing the characters of the E chinodermata, I lmve THE ANNULOIDA AND ODON'rOPHORA. 79 described at length the ambulacral system; and, in speaking of the Scolecida, I have no less insisted upon the peculiarities of the "water-vascular system." But it is impossible to compare these two systems of vessels without being struck by their similarity. Each is a system of canals, opening externally, and ciliated within; and the circumstance that the two apparatuses are turned to different purposes in two distinct groups of the animal kingdom, seems to me no more to militate against their homology, than the respiratory function of the limbs of Phyllopod Crustacea militates against the homology of these limbs with the purely locomotive appendages of other Crustaceans. Thus it appears that the Echinodermata and the Scolecida are so closely connected that they can by no means be placed in separate sub-kingdoms; and in the course of studying the other sub-kingdoms it. will be quite obvious that, unless they are to occupy an independent position, there is no place for them anywhere, save among the Annulosa. I have hitherto been accustomed to consider them, under the name of the ANNULOIDA, as a division of this sub-kingdom; but until some structural character can be discovered by which all the Annuloida agree with the Ann~~zosa, and differ from other animals, I am much inclined to think it would conduce to the formation of clear conceptions in zoology if the Annuloida were regarded as a distinct primary division of the Animal Kingdom. If we now turn to the other column of classes of invertebrate animals (supra, p. 6), the four last on the list, viz., Cephalopoda, Pteropoda, Pulmogasteropoda, and Branchiogasteropoda, have a number of well-marked characters in ·common. In all, the nervous system is composed of three principal pairs of gangliacerebral, pedal, and parieto-splanchnic-united by commissures. All possess that remarkable buccal apparatus, the odontophore,whence I have ventured ,to propose the name of 0DONTOPHORA for the group. The circulatory and respiratory organs vary a good deal, but none are provided with double lamellar gills upon each side of the body. The Lamellibranchiata stand in somewhat the same relation to the Odontophora as the Annelida to the Arthropoda. The |