448 EMBRYOLOGY 



In later stages of the larva still other canals are developed as evagina- 

 tions of the water-vascular ring and the peritoneum which covers it. 

 They grow out toward the body-wall and unite with it. At the time 

 when the larva detaches itself, five such canals are present, all of which 

 communicate with the outside world. Afterwards, however, the forma- 

 tion of the evaginations and the pores of the cup is said no longer to 

 take place simultaneously, so that the former may multiply independently 

 of the latter, and vice veraa. Thus a condition would arise such as was 

 described by Ludwig, — cup-pores leading to the interior, and free appen- 

 dages of the water-vascular ring opening into the body cavity. The canal 

 which was first formed attains an extraordinary development, and 

 Peruier regards it alone as homologous to the stone canal of other 

 Echinoderms, those formed later being of a secondary nature. 



The conditions of the body cavity of the Antedon larva are 

 complicated. The two coelomic sacs, which at first lie at 

 the iMght and left of the intestine, subsequently, when the 

 larva passes into the pentacrinoid stage, arrange themselves 

 above and below the intestine (Figs. 218 and 219). Perrier 

 desio-nates the parts of the body cavity as the subambulacral 

 and visceral portions. Where the two come together there 

 is produced a mesentery, extending transversely across the 

 body (Figs. 219 and 220). Furthermore, according to BuRY, 

 two longitudinal mesenteries are formed, owing to the facts 

 that the two coelomic sacs are (in cross-section) nearly 

 horseshoe-shaped, and that the two arms of each sac grow 

 out toward each other. In each case they meet in a longi- 

 tudinal mesentery, the one belonging to the upper entero- 

 coele lying in the anal radius and that of the lower (visceral) 

 enterocoele in the preceding radius (reckoned according to 

 the coarse of the intestine). As the coelomic sacs enlarge 

 they apply themselves to the intestine and water-vascular 

 ring as the splanchnic layer, and to the mesenchymatous 

 tissue of the body-wall as the somatic layer. The aboral 

 body cavity, as was pointed out by Goette, sends a process 

 into the narrow posterior part of the lai'val body (Fig. 220). 

 According to Perrier, this process consists of both layers of 

 the me.soderm (Fig. 219), and Goette's conjecture is con- 

 firmed, viz. that the chambered organ, which in the adult 

 animal lies within the centrodorsal plate as an important 

 part of the blood-vascular system, arises from this posterior 



