LACINULARIA SOCIALIS 14I 



I believe that there is a fundamental error in each case, namely, 

 that of approximating the Polyzoa and the Rotifera at all. The 

 resemblance between Stephanoceros and a Polyzoon is very superficial. 

 No Polyzoon has the cilia on its tentacles arranged like those of 

 Stephanoceros ; nor has any a similarly-armed gizzard : still less is 

 there any trace of the water-vascular system which exists in all 

 Rotifera. 



The relations between the Polyzoa and the Rotifera, then, are at 

 the best mere analogies. 



On the other hand, the general agreement in structure between the 

 Rotifera and the Annuloida — under which term I include the Annelida, 

 the Echinoderms, Trematoda, Turbellaria, and Nematoidea — is very 

 striking, and such as to constitute an unquestionable affinity.' 



The terms of resemblance are these : — 



1. Bands of cilia, resembling and performing the functions of the 

 wheel-organs, are found in Annelid, Echinoderm, and Trematode larvae. 



2. A water-vascular system, essentially similar to that of the 

 Rotifera, is found in Monoecious Annelids, in Trematoda, in Turbellaria_ 

 in Echinoderms, and, perhaps, in the Nematoidea.^ 



3. A similar condition of the nervous system is found in Turbellaria. 



4. A somewhat similarly-armed gizzard is found in the Nemertidae ; 

 and the pharyngeal armature of a Nereid larva may well be compared 

 with that of A Ibertia. 



5. The intestine undergoes corresponding flexures in the Echino- 

 derm larvae. There are, therefore, no points of their organization in 

 which the Rotifera differ from the Annuloida ; and there is one v&ry 

 characteristic circumstance, the presence of the water-vascular system, 

 in which they agree with them. 



Now, with what Annuloida are the Rotifera most closely allied ? 

 To determine this point, we must ascertain what is the fundamental 

 type of organization of the Rotifera. 



Suppose in Lacinularia a line to be drawn from the mouth to the 

 anus, and that this be considered as the axis of the body ; suppose 

 again, that the side on which the ganglion lies is the dorsal side, the 

 opposite being the ventral ; suppose, also, the mouth end to be 

 anterior, the anal end posterior, — then it will be found that the lower 

 circlet of cilia upon the trochal disc encircles the axis of the body, 



^ M. Milne-Edwards, with his accustomed acuteness, pointed out (Annales des Sciences, 

 1845) the close affinity of the Rotifera with the Annelids, the Turbellaria, and the 

 Xeniiatoidea ; but he did not include the Echinoderms in the group, doubtless because, at the 

 time he wrote, sufficient was not known of the Echinoderm larvce to demonstrate their truly 

 annuloid nature. 



'^ To these may be added the Cestoidea and the Nemertidce. 



