594 



COMPARATIVE ANATOMY 



CHAP. 



The region of the body lying behind this occupies in both a large part of the dorsal 

 posterior side of the body ; in it lies the anus. 



2. Whereas, in Tornaria, in the postoral area of the body, a large ciliated 

 ring bounds an anal area, such a ring is wanting in Bi])innaria. 



3. The apical plate with the two eyes and the tuft, which is so sharply marked 

 in Tornaria, is wanting in the developed Bipimiaria. Too great a significance 

 must not now, however, be attributed to this fact, since something like a neural 

 plate (an ectodermal thickening with long cilia) has been observed in the quite 

 young larvae of Asteroids and Eehinoids, and a neural plate with a layer of nerve 

 fibres, ganglion cells, and ciliated tuft, although without eyes, has been demon- 

 strated in the apical region of the Antedon larva. 



4. The intestine of Tornaria shows the same divisions as are found in that 

 of the Echinoderm larvEe, viz. : oesophagus, stomach -intestine, and hind -gut. 

 Whether, however, these three sections correspond with one another in the two 



B 



Fio. 408.— A, B, 0, Aurloularia, Blpinnaria, and Tornaria (Dnteropneustan larva), from the 

 right side, diagrammatic. 1, Preoral area ; 2, oral area ; 3, postoral area ; 4, anal area ; I, preoral ; 

 II, circumoral ; III, anal or principal ciliated ring ; 5, neural plate ; os, mouth ; an, anus. 



groups must remain uncertain so long as the origin of the oesophagus and hind- 

 gut is not definitely ascertained. In this matter the Echinoderms are in the same 

 position as the Enteropneusta. 



The cesophagus, in the Echinoderms, is sometimes described as an ectodermal 

 stomodfeum, sometimes as a section of the archenteron. The latter, according to 

 the most recent investigations, is tire case, e.g., in the Holotliurioidea, and the 

 former in Antedon and others. In this case the cesophagus, even within the 

 Eehinodermata, is not an homologous structure ! In the case of the Enteropneusta, 

 in the interest of other views, doubt has been thrown upon the statement that the 

 larval oesophagus is a part of the archenteron. 



The hind-gut in the Eehinodermata is, by all authorities, considered to be 

 endodermal. The same was aifirmed of the hind-gut of the Enteropneusta, but 

 this has recently been strongly doubted. 



5. The condition of the ocelom in the two larval forms would show great agree- 

 ment if two pairs of ccelomic sacs can really be attributed typically to the Echino- 

 derm larva, a point which recent research makes more and more probable, and if 

 also the endodermal origin of the coeloniic vesicles of the Enteropneusta could be 

 proved. 



It would then be evident that the two anterior coelomic sacs of the Echinoderm 

 larvs (the left of which is the hydrocrel) correspond with the two orelomic sacs of 



