CLASSIFICATION OF THE ANIMAL KINGDOM. 215 



shoe-shaped sac connected with the exterior by a tube which is 

 converted into the madreporic canal. Agassiz does not explain 

 the mode of formation of the perivisceral cavity of the starfish, 

 and has nothing to say concerning the origin of the pseudheemal 

 vessels. 



Eecently MetschnikofF has confirmed the observations of 

 Agassiz, so far as the development of the ambulacral system from 

 one of the diverticula of the alimentary canal of the starfish larva 

 is concerned ; and he has added the important discovery that the 

 perivisceral cavity of the Echinoderm is the product of the rest 

 of these diverticula. Moreover his observations on other Echi- 

 nodermata show that essentially the same process of development 

 of the peritoneal cavity occurs in Ophiuridea, Echinidea, and 

 .Holothuridea. 



The precise mode of origin of the pseudhsemal system, or so- 

 called blood-vessels, of the Echinoderms is not yet made out. 

 :But it is known that the cavity of these vessels contains cor- 

 puscles similar to those which are found in the perivisceral 

 cavity and in the ambulacral vessels, and that all of these com- 

 municate together. 



Agassiz and Metschnikoff alike, justly insist upon the cor- 

 respondence in development of the lateral gastric diverticula of 

 the Echinopcedium with that of the trunks of the gastrovascular 

 system of the Ctenophora ; and, on the ground of this resemblance, 

 the former refers the Echinoderms to the Eadiata, retaining under 

 that Cuvierian denomination the Acalephae (Coelenterata) and the 

 Echinodermata. But this arrangement surely ignores the great 

 value of his own discovery, which shows that the Echinoderms have 

 made a great and remarkable progress in passing from their pri- 

 marily coelenterate stage of organization to their adult condition. 

 And it further ignores the unquestionable fact, admirably brought 

 out by the same able naturalist's investigations into the develop- 

 ment of BaJanoglossus, that the Echinopaedium is almost identical 

 in structure with the young of animals, such as the Grephyrea and 

 Enteropneusta, which are in no sense radiate, but are eminently 

 bilaterally symmetrical. In fact, the larva of JBalanoglossus, the 

 sole representative of the Enteropneusta, was originally described 

 by Miiller under the name of Tornaria, as an Echinoderm larva, 

 and was subsequently more fully examined by Prof. Alex. Agassiz, 

 who also regarded it as an unquestionable Echinoderm larva; 

 and it is only recently that it has been proved, partly by Metsch- 



