78 EMBRYOLOGY OF THE STARFISH. 



simply a means of locomotion for the young Starfish till it shall have 

 acquired a sufficient size to be able to take care of itself, and use its 

 suckers as organs of locomotion. 



Have we not here, in Echinoderms, something analogous to what we 

 have in Discophorous Medusa? ? In Cyanea and Pelagia, for instance, 

 where, in one case, the young Acaleph passes through a Scyphistoma 

 stage before it reaches the Ephyra condition, while in Pelagia, on the 

 contrary, the Ephyra is at once produced from the egg, without passing 

 through the Scyphistoma stage. 



I think it can be easily shown that there is, in reality, no difference be- 

 tween these two modes of development ; it is merely a question of quantity. 

 In Cribrella, in Pluteus, in Brachiolaria, or in Auricularia, the young Echi- 

 noderm is developed on the outer surface of the water-system. The 

 water-tubes obtain a great prominence in Auricularia, in Brachiolaria, 

 and in the Pluteus-like form of the Ophiurans and Echini, while in types 

 of development like those of Echinaster they remain more rudimentar}' ; 

 the only appendages developed in this last type being those which cor- 

 respond to later periods of growth in the Starfish larvae, viz. the 

 brachiolar appendages. The peduncle and its appendages, by means of 

 which the young Echinaster fiistens to the rocks, are strictly homologous 

 to the brachiolar appendages of our Starfish larvae. In ftict, when the 

 young Starfish has resorbed all the arms, and there is nothing left of 

 them, except a few swellings on the actinal side, to mark their former 

 position, the brachiolar appendages are in exactly the same position 

 as that occupied by the peduncle of the Echinaster larva. Had we known 

 nothing of the previous modes of development, and found those young 

 Starfishes in the open sea in this stage, nothing -would have been more 

 natural than to have assumed that they had reached this condition by the 

 same mode of development. The cavity noticed in the peduncle of the 

 Echinaster larvae is part of the water-system, corresponding to the branch 

 of the water-system leading into the brachiolar arms of our Asteraoan- 

 thion larva. 



The same is the case with the two modes of development of Ophiurans 

 and of Holothurians ; they are shorter ways of arriving at the same point, 

 wliether they pass through what we shall call hereafter the Pluteus type 

 of development or the Echinaster type ; in either of the orders it is one 

 and the same tiling differently carried out. The larviD of our Cribrella, 



