ECHINODERMA GENERAL DESCRIPTION 13 



of zoologists. The development of Asterina indicates the possible 

 relation between those two groups. Here MacBride (1896) has 

 shown that the larva is early attached by the preoral lobe, and 

 that it bends over on this so as to bring the mouth downwards. 

 The internal structures do not, however, undergo that complete 

 translation and torsion which occur in the Crinoid. Traces of it 

 are seen in the greater development of the left hydrocoel and left 

 posterior coelom. Fig. XI. attempts to show what would happen 

 in the case of a primitive Pentadcea that bent over in this way ; 

 while the mouth passed down, the anus and hydropore would tend 

 to remain on the upper surface, where they could best fulfil their 

 functions. In the present ontogeny of the Asteroid, the develop- 

 ment is direct from the Dipleurula to this stage, the intermediate 

 steps imagined for the phylogeny being omitted as unnecessary. 



Fio. XI. 



Change from Pentactcea to Stelleroid type. 0, mouth ; As, anus ; M, madreporite ; r.p.c 

 and l.p.c, right and left posterior coelom ; l.hc, left hydrocoel ; s.c, stone canal ; pi, preoral 

 lobe ; ax, axial sinus, remains of anterior coelom. 



But to those phylogenetic steps are due the peculiar positions 

 assumed by the left hydrocoel and left posterior coelom, as well 

 as the radial folding which they undergo. Study of Fig. XI. 

 will elucidate the complicated internal arrangement of the develop- 

 ing Asterina, Further flexure causes the ends of the curved 

 hydrocoel to grow around the stalk, which thus deceptively appears 

 to spring from the oral surface, not from the aboral as in Crinoids. 

 Subsequently the stalk atrophies, and the young starfish is a free- 

 moving form, with mouth on the sea-floor, with anus and madre- 

 porite directed upwards, and with the beginnings of five arms 

 containing extensions of the left hydrocoel, of the oral coelom 

 (derived from 1. post, coelom), and of the stomach (Fig. XII.). 

 During development the larval mouth and anus are closed, and 

 break through again in their adult positions ; this points to a 

 migration of those openings durinr, phylogeny, which migration 

 cannot well be repeated in ontogeny. 



A vast amount of discussion has taken place over the question 

 whether the plates of the Crinoid calyx find homologues in other 



