THE PYCNOGONIDS. 29 



Dohrn has ably maintained the first theory, and the preceding 

 translation gave the conclusion he had reached. Dr. Hoek 

 likewise, as we have seen, holds this opinion, although not 

 agreeing as to details with Prof. Dohrn. On the other hand, a 

 study of the early stages of the embryology has brought to light 

 certain facts which for me point decidedly toward a community 

 of descent between Arachnids and Pycnogonids. The latter 

 show undoubted traces of degeneration, and we cannot derive 

 them from any existing animals. But I believe the Pycno- 

 gonids and the Arachnids have come down along the same line, 

 or, in other words, have had ancestors in common long after 

 those ancestors came from Annelid-like forefathers. The reasons 

 for such a belief are as follows : 



1. The Pycnogonids form the endoderm by a process of multi- 

 polar delamination, which is shown in its simplest form in 

 Phoxichilidium and Tanystylum, and in a more modified con- 

 dition in Pallene. In no other group of the Triploblastica do 

 we find a similar phenomenon except in the Arachnids. The 

 description of the embryology of Chelifer which Metchnikoff has 

 given shows this process or something quite analogous to take 

 place in it. The segmentation is holoblastic, and at a later 

 stage the large cells containing yolk divide into an outer, more 

 protoplasmic layer of cells and the inner cells, which are very 

 granular. The outer form the ectoblast, and most probably the 

 inner, judging from his figures, form the entoblast. 



In the Spiders the process is not so well marked, but if the 

 conception which Balfour had of the formation of the yolk- 

 nuclei be true, then we may make a direct comparison between 

 the two groups. He says : " It appears to me probable that at 

 the time when the superficial layer of protoplasm is segmented 

 off from the yolk below the nuclei undergo division, and that a 

 nucleus with surrounding protoplasm is left with each yolk 

 column." This description for the Spiders may be substituted, 

 word for word, for the process of delamination of Pallene. 



2. The first trace of the embryo to appear in Pallene is a 

 round opaque area at the spot where the stomodseum invag- 

 inates. In Schimkewitsch's recent account for the Spiders he 

 shows that the primitive cumulus in them is the place where the 

 stomodseum invaginates. This is also true for Pallene, but here 



