EC HI NO DERM A TA . 465 



paired dorsal arm, and the two ventral oral arms. The extent of the 

 development of the arms varies with the species. 



.4. 



FIG. 259. DIAGRAMMATIC REPRESENTATION OF VARIOUS FORMS OF ASTEROID LARWE. 

 A, B, C, BIPINNARIA ; D, BRACHIOLARIA. (Copied from Miiller.) 



The black lines represent the ciliated bauds ; and the shading the space between 

 the pra3-oral and the post-oral bands. 



HI. niouth ; an. anus. 



The changes by which the Bipinnaria or Brachiolaria becomes 

 converted into the adult starfish are very much more complicated 

 than those which take place in Holothurians. For an accurate know- 

 ledge of them we are largely indebted to Alex. Agassiz (No. 543). 

 The development of the starfish takes place entirely at the posterior 

 end of the larva close to the stomach. 



On the right and dorsal side of the stomach, and externally to the 

 ritjht peritoneal space, are formed five radially situated calcareous 

 rods arranged in the form of a somewhat irregular pentagon. The 

 surface on which they are deposited has a spiral form, and consti- 

 tutes, together with its calcareous rods, the abactinal or dorsal 

 surface of the future starfish. Close to its dorsal, i.e. embryonic 

 dorsal, edge lies the dorsal pore of the water- vascular system (madre- 

 poric canal), and close to its ventral edge the anus. On the left and 

 ventral side of the stomach is placed the water-vascular rosette, the 

 development of which was described on p. 547. It is situated on the 

 actinal or ventral surface of the future starfish, and is related to the 

 left peritoneal vesicle. 



Metschnikoff (No. 560) and Agassiz (No. 543) differ slightly as to the 

 constitution of the water- vascular rosette. The former describes and figures 

 it as a completely closed rosette, the latter states that 'it does not form a 

 completely closed curve but is always open, forming a sort of twisted 

 crescent- shaped arc.' 



The water-vascular rosette is provided with five lobes, correspond- 

 ing to which are folds in the larval skin, and each lobe corresponds 

 to one of the calcareous plates developed on the abactinal disc. 



B. E. 30 



