THE RELATIONSHIPS OF THE SEA-SPIDERS. 1 53 



mental characteristics, will not of necessity include 

 within it the Pycnogonids. We have the segmented 

 body with its segmented appendages, typically a pair 

 to each segment. There is a supra-oesophageal ganglion 

 — the brain — connected by commissures with a double 

 ventral chain of ganglia. Digestive tract, circulatory 

 apparatus, and to some extent^ the generative organs, 

 fall into line with the other Arthropod characteristics. 



Comparing the adult structures of the sea-spiders 

 with the Arachnids, we find many common characteris- 

 tics ; and I must point out that many of these similarities 

 are in structures which we must believe the primitive 

 ancestors of the Arachnids to have had more fully de- 

 veloped than the living forms to-day. 



First, we may consider the digestive tract. In the 

 adult sea-spiders we find long diverticulae from the mid- 

 gut into the appendages ; and we find in the early stages 

 of development of both (false) scorpions and spiders 

 similar pouches. ^ The heart, too, with its lateral ostia 

 resembles that of spiders, though the resemblance is 

 not more striking than to Arthropods in general, and I 

 have spoken above of the resemblance between the 

 chelae and their innervation in the sea-spiders and 

 Arachnids. Further, the external openings of the re- 

 productive organs in Limulus lie on the base of the first 



1 The opening of the ducts of the reproductive organs are not as 

 difficult to account for as I had at first supposed. We may imagine them 

 to have secondarily shifted from the body to the legs, and we see in 

 Limulus (" An Arachnid ") the external reproductive orifices on the base 

 of the first pair of abdominal appendages. 



2 The structure and arrangement of the so-called livers of Limulus and 

 Scorpions is interesting, inasmuch as they may represent ancestral ab- 

 dominal pouches. 



