ECHINODERMATA. 559 



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 (madreporic canal), and close to its ventral 

 edge the anus. On the left and ventral side of the stomach is 



o 



placed the water-vascular rosette, the development of which was 

 described on p. 549. 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, corre- 

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

 corresponds to one of the calcareous plates developed on the 

 abactinal disc. The plane of the actinal surface at first meets 

 that of the abactinal at an acute or nearly right angle. The two 

 surfaces are separated by the whole width of the stomach. The 

 general appearance of the larva from the ventral surface after 

 the development of the water-vascular rosette (i) and abactinal 

 disc (A] is shewn in fig. 260. 



As development proceeds the abactinal surface becomes a 

 firm and definite disc, owing to the growth of the original 

 calcareous spicules into more or less definite plates, and to the 

 development of five fresh plates nearer the centre of the disc and 

 interradial in position. Still later a central calcareous plate 

 appears on the abactinal surface, which is thus formed of a 

 central plate, surrounded by a ring of five interradial plates, and 

 then again by a ring of five radial plates. The abactinal disc 

 now also grows out into five short processes, separated by five 

 shallow notches. These processes are the rudiments of the five 

 arms, and each of them corresponds to one of the lobes of the 

 water-vascular rosette. A calcareous deposit is formed round 

 the opening of the water-vascular canal, which becomes the 

 madreporic tubercle 1 . At about this stage the absorption of the 

 larval appendages takes place. The whole anterior part of the 



1 The exact position of the madreporic tubercle in relation to the abactinal plates 

 does not seem to have been made out. It might have been anticipated that it would 

 be placed in one of the primary interradial plates, but this does not seem to be the 

 case. The position of the anus is also obscure. 



