Note on the Development of Cribrella oculata. 313 



their stalk, and become free. The development of Cribrella, 

 though, as in Asterina, apparently favouring Macbride's view, 

 which would lead us to suppose the Crinoids and Asterids to 

 be widely divergent, seems nevertheless to furnish some 

 direct evidence to the contrary conclusion, and for reconciling 

 the ontogeny of Asterina and Antedon, Lastly, there occur 

 certain specimens which are enantiomorphs of the normal. 

 The right hydrocoele forms the ambulacral system, and so on. 

 When we consider that with the destruction of the preoral 

 lobe all evidence of the peculiarity of these specimens is 

 removed, one is led to believe that this may be of compara- 

 tively common occurrence in natural conditions, as in 

 flounders. 



Figs. 5 and 6 are meant to indicate that it is possible to 

 draw as close a comparison between the demersal echinoderm 

 larva and the young Balanoglossus as between the pelagic 

 echinoderm larva and Tornaria. 



Explanation of Figures. 



Fig. 1. Coronal section through young larva, showing blastoporic pit and 

 vesicular archenteron. 



Fig. 2. Coronal section through later stage. The archenteron has given off 

 at the front end a large anterior ccelom. 



Fig. 3. Coronal section through rather later larva. The two hydrocceles are 

 becoming constricted off from the rest of the anterior ccelom, 

 which will form the preoral ccelom, contained in the preoral lobe. 

 The posterior ccelom is arising from the hind end of mesenteron. 



Fig. 4. Dorsal view of three-armed larva, seen as transparent object. The 

 left and right enterocceles grow forwards ventrally and the left 

 dorsally; not shown here, but seen in Fig. 6. The preoral sac 

 is arising from preoral ccelom to right side of middle line. 



Fig. 5. Diagram of young Balanoglossus (collar-pores omitted). Dorsal 

 view. 



Fig. 6. Diagram of late Cribrella larva, before fixation. The hydropore does 

 not in reality perforate the ectoderm till later, but is in this posi- 

 tion in Asterina and others. Dorsal view. 



