I 82 CASWELL GRAVE. 



have to do with highly specialized organisms, there being a long 

 period in the life of each during which it is thrown upon its 

 own resources. Its existence during this period depends upon 

 its ability to procure food and escape from its enemies. The 

 rigorous selection which must take place under these conditions 

 can not have failed to have had a profound effect upon the whole 

 organization of the larvae and especially upon their external 

 characters. 



The whole tendency has been however to look for primitive 

 characters among free-swimming larvae, throwing aside those 

 which are brooded or otherwise cared for as much more likely to 

 be modified and secondary. If my suggestion as to the signifi- 

 cance of the larvae of echinoderms with transverse rings is correct 

 then this view is incorrect. On the other hand we would expect 

 to find the least modified development among larvse which are 

 freed, to a greater or less extent, from the task of caring for 

 themselves, provided in such cases the eggs have not been so 

 crowded with nutritive materials as to become greatly enlarged 

 or that, during the brooding, no connections with the mother are 

 established or protective structures developed. Kenogenetic 

 characters are no doubt found in both types of larvae and the 

 problem is to ascertain which has remained truer to the ances- 

 tral form. 



The final, sudden and complicated metamorphosis into the 

 adult form which is so characteristic c& free-swimming larvae is 

 good evidence that they have been carried far out of the path of 

 phylogeny. In larvae without a long independent existence the 

 metamorphosis is gradual and such as might be expected if it is 

 in any way a true picture of the past history of the race. 



No very great similarity is shown in the external forms of the 

 familiar types of echinoderm larvae and it is difficult to think of 

 any one of them as having been the type from which the others 

 originated, but it is possible to think of them all as having arisen 

 from a type of larva such as is found in Antedon, Cucumaria and 

 Ophiura. 



Owing to the similarity in position of the ciliated rings with 

 reference to the other organs of the body of the larvae of the 

 above-named species, and all other cases in which ciliated rings 



