E1PINNARIA. 



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 (%] 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 abactiual disc now also grows out into five short pro- 

 cesses, 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 madre- 

 poric tubercle 1 . At about this stage the 

 absorption of the larval appendages takes 

 place. The whole anterior part of the 

 larva with the great pra-oral lobe has 

 hitherto remained unchanged, but now it 

 contracts and undergoes absorption, and 

 becomes completely withdrawn into the 

 disc of the future starfish. The larval 

 mouth is transported into the centre of 

 the actinal disc. In the larvae observed 

 by Agassiz and Metschnikoff nothing was 

 cast off, but the whole absorbed. 



According to Muller and Koren and 

 Danielssen this is not the case in the larva 

 observed by them, but part of the larva is 

 thrown off, and lives for some time inde- 

 pendently. 



After the absorption of the larval ap- 

 pendages the actinal and abactinal surfaces 

 of the young starfish approach each other, 

 owing to the flattening of the stomach ; at 

 the same time they lose their spiral form, 

 and become flat discs, which fit each 



FlG. 260. BlPINNARIA LAKVA 



OF AN ASTEROID. (From Ge- 

 genbaur ; after Muller. ) 



b. rnouth; a. anus; 7*. ma- 

 dreporic canal; i. ambulacral 

 rosette; c. stomach; d. g. e. 

 etc. arms of Bipinnaria ; A. 

 abactiual disc of young Asteroid. 



other. Each of the lobes of the rosette 



of the water-vascular system becomes one of the radial water- 

 vascular canals. It first becomes five-lobed, each lobe forming 



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. 



