478 COMPARISON OF ECHINODERM LARV.E. 



From the edge of the vestibule the arms grow out, carrying 

 with them the tentacular prolongation of the water-vascular ring. 

 Two additional rows of radial s are soon added. 



The stalked Pentacrinoid larva becomes converted, on the absorp- 

 tion of the stalk, into the adult Antedon. The stalk is functionally 

 replaced by a number of short cirri springing from the centro-dorsal 

 plate. The five basals coalesce into a single plate, known as the 

 rosette, and the five orals disappear, though the lobes on which 

 they were placed persist. In some stalked forms, e.g. Rhizocrinus 

 Hyocrinus, the orals are permanently retained. The arms bifurcate 

 at the end of the third radial, and the first radial becomes in Antedon 

 rosacea (though not in all species of Antedon) concealed from the 

 surface by the growth of the centro-dorsal plate. An immense number 

 of funnels, leading into the body cavity, are formed in addition to the 

 single one present in the young larva. These are regarded by Ludwig 

 as equivalent to so many openings of the madreporic canal ; and 

 there are developed, in correspondence with them, diverticula of the 

 water-vascular ring. 



Comparison of Echinoderm Larvce and General Conclusions. 



In any comparison of the various types of Echinoderm larva? 

 it is necessary to distinguish between the free-swimming forms, and 

 the viviparous or fixed forms. A very superficial examination suffices 

 to shew that the free-swimming forms agree very much more closely 

 amongst themselves than the viviparous forms. We are therefore 

 justified in concluding that in the viviparous forms the development 

 is abbreviated and modified. 



All the free forms are nearly alike in their earliest stage after 

 the formation of the archenteron. The surface between the anus 

 and the future mouth becomes flattened, and (except in Antedon, 

 Cucumaria, Psolinus, etc. which practically have an abbreviated 

 development like that of the viviparous forms) a ridge of cilia 

 becomes established in front of the mouth, and a second ridge 

 between the mouth and the anus. This larval form, which is shewn 

 in fig. 264 A, is the type from which the various forms of Echinoderm 

 larva? start. 



In all cases, except in Bipinnaria, the two ciliated ridges soon 

 become united, and constitute a single longitudinal post-oral ciliated 



ring. 



The larva? in their further growth undergo various changes, and 

 in the later stages they may be divided into two groups : 



(1) The Pluteus larva of Echinoids and Ophiuroids. 



(2) The Auricularia (Holothuroids) and Bipinnaria (Asteroids) 

 type. 



plates to the primitive peritoneal vesicles, and I am inclined to believe that this method 

 of dealing with these homologies is the right one. Ludwig (No. 559) by regarding the 

 opening of the madreporic canal as a fixed point has arrived at very different results. 



