62 EMBRYOLOGY OF THE STARFISH. Part I. 



of development in the Starfishes, the one as observed by Sars and Agassiz, in 

 Echinaster, the other in which the embryo assumes the shape of a Bipinnaria or 

 Brachiolaria ; and, finally, in the Holothurians we have these two modes represented 

 by the Auricularia type, and the type of the " Wurmformige Holothurienlarve." The 

 difference between these two modes seems to be one of time ; in one case, the 

 eggs are retained by the parent until they have passed through many of their 

 changes, and are freed in a stage corresponding to that of our young Echino- 

 derm after it has resorbed its Pluteus, its Brachiolaria, or its Auricularia. In 

 the other case, the egg goes through all these changes after it has left the 

 parent, developing this complicated system of arms, which seems to be 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 Medusas ? 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 in these two 

 modes of development ; it is merely a question of quantity. In Cribrella, in Pluteus, 

 in Brachiolaria, or in Auricularia, the young Echinoderm is developed on the outer 

 surface of the water-system. The water-tubes obtain a great prominence in Auricula- 

 ria, 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 rudimentary ; the 

 only appendages developed in this last type being those which correspond to later 

 periods of growth in the Starfish larva?, — viz. : the brachiolar appendages. The pe- 

 duncle, and its appendages, by means of which the young Echinaster fastens to the 

 rocks, are strictly homologous to the brachiolar appendages of our Starfish larva?. In 

 fact, 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 at once in this stage, nothing 

 would have been more natural than to have assumed that they had arrived at this 

 stage by the same mode of development. The cavity, noticed in the peduncle of 

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

 water-system leading into the brachiolar arms of our Asteracanthion 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, whether they 

 pass through what we shall call hereafter the Pluteus type of development, or the 



