HOMOLOGIES AND AFFINITIES OF ECHINI. 76I 



rians and some of the Starfishes (Cuvieria and Cribrella) have an analo- 

 gous mode of development. The Auric ularians (Synapta) are most closely 

 allied, as far as the pluteus is concerned, to the Bipinnaria, and Brachiolaria 

 (Asteracanthion). The pluteus of the Ophiurans is, on the contrary, most 

 closely allied to that of Echinoids, while, as far as the Comatulae are con- 

 cerned, they recall in their general features some of the Holothurian embryos. 



As far as the affinities of the Echinoderms with Worms are concerned, the 

 recent observations on Comatula, made by Metsclmikoff, appear to strengthen 

 the views of Huxley* and Haeckel, which are, however, entirely based on 

 theoretical considerations.! The discovery of the final development of Tor- 

 naria, which till lately was supposed to be a starfish pluteus, has shown con- 

 clusively that the whole plan of its development is entirely different from 

 that of Echinoderms. Although we have a water-system in Tornaria, it is 

 not bilateral, but is dorsal, and the presence of a true heart shows that it has 

 nothing in common with the water-system of Echinoderms ; the transforma- 

 tion into the Balanoglossus takes place by a simple change of topography 

 in the organs of Tornaria, while in Echinoderms we have a gradual resorption 

 into the young Echinoderm of the whole of the Brachiolaria in Starfishes, of 

 the Auricularia in Synapta, and of the pluteus in Echinoids and Ojmiurans, 

 though the development of the Echinus and Ophiuran does not seem to de- 

 pend entirely upon this resorption, which does not always take place com- 

 pletely and is frequently accompanied by a greater or less previous decom- 

 position of the arms. 



As far as the early embryonic stages of the Echinoderms, Acalephs, and 

 Polyps are concerned, the formation of the digestive cavity by the turning in 

 of the outer wall of the embryo is the same in all, and in Ctenophorae the 

 digestive cavity has walls fully as thick in the planula stage as those we 



* For a discussion of those views, see my paper on Tornaria, Mem. Am. Acad., 1873. 



f Of course it must be remembered, in making this comparison, that the Annulata are by no means a 

 homogeneous group. The true Annelids are undoubtedly most closely related to the Crustacea and Insects, 

 while the Gephyreans, Planarians, Xemerteans, and perhaps other Annulata, are more nearly allied to the 

 Echinoderms, both from the nature of their embryological development and from the presence of a more 

 or less complicated water-system, analogous to that of Radiates. 



Systematists are evidently drifting back to some of the exploded (?) views, formerly held by Cuvier and 

 till comparatively lately by Forbes, of the relationship of the Gephyreans with the Echinoderms. It may 

 be, therefore, that the splitting up of the Annulata into two branches, the one (the Annelids proper and 

 their allies) closely allied to the Arthropods, the other (the Gephyreans, Xemerteans, and the like) more 

 closely related to the Echinoderms, may explain many doubtful points regarding the affinities of the Annu- 

 lata as usually understood. 



