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Show 76 ON OLASSlFICATION. 1. espect s, Annelids differ wid.e ly. frmn .dA rft hro. pods. It n1ay 1 . . b e d ou bte d whether any Annelid IS devm o . CI 1a 1n some part 1 . or other of its organization, and cilia constitute t 1e most In1- portant organs of locomotion in ~he. embryos of many. No A lid possesses a heart con1mun1cating by valvular apertures ·w inthn e the perivisceral cavity, none have artJ· cu1.n tc d 11· n~ bs , an d none possess a head composed of ev~n four moddi~d somites. Most Annelids are provided w1th that pecuhar system of vessels termed "pseudo-hremal ;" but, in some, that system has not yet been discovered. . In endeavouring to separate from among Invertebrated animals a first large group, con1parable to the Vertebrata, it appears to me that the rese1n~lances between the Annelida find the Arthropoda outweigh the differences ; and that the c-haracters of the nervous system and the frequently segmented body, with imperfect lateral appendages, ~f the former, n:-~e~sitate their assemblage with the Arthropoda Into one great divisiOn, or " sub-kingdom," of ANNULOSA.. But what of the Echinodermata and the S colecida? Should both these great classes be als.o ranged under the Annulosa; or do they belong to different sub-kingdoms ; or, if they belong to the same, should they constitute a sub-kingdmn of their own? I will endeavour to reply to these questions in succession. vVhether these two groups belong to the Annulosa or not, must depend upon whether they poR ess any characters in common with the Arthropoda and Annelida oth r than those which they have in cmnmon with all animals. I can find none of any great n1oment. No Echinoderm, or Scolecicl, has a definitely segmented body or bilaterally disposed successive pairs of appendages. None of these anin1als ha. a longitudinal chain of ganglia. On the other hand there is much resemblance between the ciliated larvre of some' Scolecids and Echinoderms, [tnd those of Annelids; and the form of the body of many Scolecids is so similar to that of one of the most familiar of Annelids, as to have earned for both them and the Annelids the common title of "worms." Nor must it Le forgotten that, in the Annelids,. there seem to be representatives of that singular system of THE ANNULOIDA. 77 vessels which attains so large a development as tho "water-vascular " apparatus in many Scolecids. . vVhatever value may be attached to these resemblances, It must, I think, be admitted that, in the present state of our knowledo·e it is impossible to affirm anything absolutely common 0 to, and yet' diagnostic of, all Annulosa and all Echi• nod e rms and Scolecids. On the other hand, there can be no doubt as to the many and singular resemblances which unite the Scolecids an~ the Echinoderms together. The nervous system of the Echinoderm may present considerable differences from that of a Trematode or l{otifer, but it must be recollected that the comparison is not a fair one, seeing that the mouth and gullet of an Echinoderm, round which its nervous ganglia are arranged, are not, strictly speaking, the same as the paris so named in a Rotifer, bnt are new developments. And it is exactly in that anomalous method of development of the Echinoderm within its larva, which is so characteristic of the whole group of Echinodermata, that this class exhibits its strong alliance with the Scolecida; the Turbellaria and Tmniada exhibiting the only approach to the method of Echinoderm development known in the Animal Kingdom. A singular larva studied by Johannes Muller, in one of his rnany fruitful visits to the seashore, and termed by him Piliclium, has furnished, in the hands of subsequent observers (more especially Krohn, Leuckart, and Pagenstecher), ample proof that a Nemertes (a genus of Turbellaria) may be developed in a manner altogether sin1ilar to that in which an Echinoderm takes its origin. The Pilidium (Fig. 37) is a small, helmet-shaped larva, with a long flagellum attached like a plume to the summit of the helmet, the edges and side lobes of which are richly ciliated. A simple alimentary sac opens upon the under surface of the body between the lobes (Fig. 37, A). In this condition, the larva swims about freely; but., after a while, a mass of formative matter appears upon one side of the alimentary canal, and, elongating gradually, takes on a worm-like figure. Eventually it grows round the alimentary canal, and, appropriating it, detaches itself from the Pilidium |