THE RELATIONSHIPS OF THE SEA-SPIDERS. 15/ 



speak later. And these points of difference in the 

 development may be mentioned. There is no primitive 

 cumulus or its homologue in the Crustacea, and the first 

 invagination of the surface ectoderm of the embryo goes 

 to form the mid-gut, and its point of origin corresponds, 

 approximately (perhaps entirely), with the permanent 

 anus. Further, there is nothing corresponding to the 

 gut-pouches so characteristic of the Pycnogonids and, 

 to some extent, of the Arachnids. 



If we turn to the insects, which form by themselves 

 so well-defined a group, we see little or nothing new for 

 comparison with the Pycnogonids.^ 



Returning again to the Arachnids, we have found that 

 the Pycnogonids have, in common with them, not only 

 the general characteristics of the Arthropods but many 

 peculiarities of development and many structures com- 

 mon in the adults. If we accept this we have gone 

 perhaps as far as the facts at present at our command 

 will allow. For it is impossible to say at just what point 

 the Pycnogonids have branched off from the phylum of 

 the Arachnids ; but, inasmuch as several of the adult 

 characteristics of the sea-spiders are found in the em- 

 bryos of higher Arachnids, we may fairly believe that 

 the group arose quite far down amongst the primitive 

 Arachnids, to have retained many of their primitive 

 structures, and to have lost others which the ancestral 

 form possessed becoming simplified, specialized, and 

 degenerate. 



1 If Heider's and Wheeler's accounts of the early formation of inner 

 yolk nuclei from the outer (ectoderm) nuclei be true, it looks exceedingly 

 like multipolar delamination for the insects. 



