560 



BIPINNARIA. 



larva with the great prae-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 becomes completely withdrawn into the disc of the future starfish.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 Met- 

 schnikoff nothing was cast off, but the 

 whole absorbed. 



According to M tiller 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 indepen- 

 dently. 



After the absorption of the larval 

 appendages 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 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 

 a rudimentary tube foot, and on each 

 side of the middle lobe two fresh ones 

 next spring out, and so on in succession. The terminal median 

 lobe forms the tentacle at the end of the arm, and the eye is 

 developed at its base. The growth of the water-vascular canals 

 keeps pace with that of the arms, and the tube feet become 

 supported at their base by an ingrowth of calcareous matter. 

 The whole of the calcareous skeleton of the larva passes directly 

 into that of the adult, and spines are very soon formed on the 

 plates of the abactinal surface. The original radial plates, 

 together with the spines which they have, are gradually pushed 

 outwards with the growth of the arms by the continual addition 

 of fresh rows of spines between the terminal plate and the plate 

 next to it. It thus comes about that the original radial plates 

 persist at the end of the arms, in connection with the unpaired 



FIG. 260. BIPINNARIA 

 LARVA OF AN ASTEROID. (From 

 Gegenbaur ; after Miiller.) 



b. mouth ; a. anus ; h. ma- 

 dreporic canal ; /'. ambulacral 

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

 etc. arms of Bipinnaria ; A. 

 abactinal disc of young Aste- 

 roid. 



