28 T. H. MORGAN. 



Annelid forms which were homonomously segmented. Then 

 the larva of the Pantopods came from a larval form with Panta- 

 pod characters added on, but at the same time a larva which 

 never had an independent and mature existence.' " 



Whatever Hoek's position may be with respect to the Panto- 

 pod-larva, he agrees with Dohrn in the most important part of 

 the latter's theory that the Pycnogonids have come down from 

 the Annelids independently of the other groups of Arthropods. 

 In examining the preceding account we may take it up for the 

 sake of clearness in two parts, the first pertaining to the ancestry 

 of the adults, leaving the larval form out of account, and the 

 second part where the meaning of the larval form will be 

 considered. 



I believe that if the account I have given of the early stages 

 of development be even approximately correct, there is little or 

 no ground for a comparison between Crustacea and Pycno- 

 gonids — certainly not with any existing forms. The multipolar 

 delamination of the endoderm in the Pycnogonids has no 

 homologue amongst the Crustacea, nor is there any special 

 similarity in the formation of the organs. There seems to be 

 no trace of gastrulation like the Crustacean in the ontogeny of 

 the group. And if we have reason for rejecting a relationship 

 between the Pantopod-larva and the Nauplius — and I believe 

 with Dohrn that we have — there remains nothing in common to 

 the ontogeny of the two groups. 



Nor are there any special affinities between the Insects and 

 Sea-Spiders, but there is one striking similarity between the 

 latter and Peripatus, which I have already spoken of; but an 

 isolated fact of this kind gives little ground for further com- 

 parisons. The ventral organs in the two groups present a 

 striking agreement, but there is no proof forthcoming as to a 

 real homology of the structures. The process of the formation 

 of the endoderm described by Heider and by Wheeler in Insects 

 shows a certain resemblance to multipolar delamination, but if 

 it be such it is a more complicated form than shown by the 

 Pycnogonids. With these two exceptions there seems to be 

 nothing else in common in the ontogeny. 



We are then left to decide between an independent origin for 

 the Pycnogonids or a relationship with the Arachnids. Prof. 



