THE RELATIONSHIPS OF THE SEA-SPIDERS. 165 



The chelae are identical with those of the adult, and pass 

 over during the metamorphosis without change into the 

 first pair of adult appendages. The ventral ganglia 

 arise in the young larva, as do the later ones, by a pecu- 

 liar process of invagination, and differ markedly from the 

 origin of similar ganglia in the Trochophore ; and lastly, 

 the ectoderm and its sense organs are the same as those 

 of the adult. Now while, as I have said, it is just pos- 

 sible all of the things (with the exception of the Procto- 

 daeum !) may, as Dohrn believes, have been thrown back 

 upon the larva, such a process seems improbable in 

 itself, and, I think, is an entirely unnecessary supposition. 



For the necessity of believing that the young forms, 

 in such groups as Annelids and Crustacea, more nearly 

 resemble each other than do the adults seems to me 

 an entirely unwarranted supposition. On the contrary, 

 I think a priori we should expect to find exactly the 

 reverse of this, — that is, that the adults are nearer 

 together ancestrally than are the larvae. 



If we stop to recall the fact that most of these ani- 

 mals lay great numbers of eggs and that almost all of 

 these are destroyed during the larval stages, and out of 

 several thousands only a few reach the adult condition, 

 then I think we must see that the battle for existence 

 amongst the larvae as compared with the adult is as a 

 thousand to one ; and hence, when there is this vigorous 

 process of natural selection going on, we must expect 

 the embryos to become changed, or adapted to new con- 

 ditions, with great ease and rapidity. 



And now if we remember that during the time in 

 which the groups of Annelids and Crustacea have been 

 evolved that the larval forms themselves have been 



