THE RELATIONSHIPS OF THE SEA-SPIDERS. 163 



the head segment. ^ Soon, however, other segments 

 appear in the anal region, so that we see at one time 

 three segments, two besides the head, and it is at such 

 a stage we may compare the Trochophore with the 

 other forms. 



The characteristics of the Pantopod-larva have been 

 already described on page 148, and reference to Fig. 2 

 may serve to recall its more important features. 



We have seen that Hoek holds the Pantopod-larva 

 to be a primary form, and that Dohrn believes it to be 

 neither primary nor secondary, but what I have called a 

 transformed larva. 



It is exceedingly difficult to combat these theories, 

 but this much must be against Hoek's position, viz. that 

 the same objections which are nowadays being brought 

 against the Nauplius theory of the Crustacea, must tell 

 at every step against his Pantopod-larval theory for the 

 Pycnogonids ; and I believe morphology is outgrowing 

 the Nauplius theory. To-day it seems to be on its last 

 legs — or rather more legs have been forced upon it 

 than it could conveniently carry. Some of its earliest 

 and most brilliant advocates, including Hatschek, Dohrn, 

 and Claus, have at last thrown it over, so that it seems 

 superfluous to repeat the same arguments against the 

 Pantopod-larva that have been brought against the 

 Nauplius theory. 



Turning now to Dohrn's theory, we cannot but grant 

 that it at least is a theoretical possibility, even if the 

 Pycnogonids be related to Arachnids. It only remains 

 to examine the facts which we have and see if they 



1 This is not quite exact, for the posterior part of this segment contains 

 the rudiment of the future body. 



