No. 2.] THE EMBRYOLOGY OF LIMULUS. 243 



must have been derived from some metameric structure 

 connecting the coelom with the exterior, and the nephridia of 

 the annelids are the most probable if not the only ducts which 

 answer the conditions. 



Limulus, the Arachnids, the Crustacea and the Chilognaths 

 agree in having the genital ducts some little distance in 

 advance of the posterior end of the body while in the Hexa- 

 pods and Chilopods they are sub-terminal, but how much 

 weight is to be given this point is not yet apparent. 



The reticulate and anastomosing character of the genital 

 ducts in Limulus and the Arachnids has been commented upon 

 by Lankester. Such conditions are not paralled in the Ar- 

 thropods except in certain Phyllopods. Again the existence of 

 motile spermatozoa in both Limulus and Arachnids and their 

 absence from all Crustacea except the Cirripedia has a certain 

 value as cumulative evidence. 



The correspondences between the circulatory systems of 

 Limulus and the scorpions are remarkably close. In both 

 there is the same median anterior aorta which divides and 

 passes downward, as a pair of sternal arteries — one passing 

 on either side of the oesophagus — which unite below in a 

 ventral vessel in close connection with the ventral nerve chain. 

 In the scorpions this ventral vessel consists of an artery 1 

 lying Jipon the nervous system, and following not only the 

 ventral cord but the various metameric nerves which arise 

 from it. This condition, which is characteristic of the adult 

 scorpion is found in the earlier stages of Limulus. Later the 

 neural artery completely envelopes the ventral cord and its 

 nerves in the manner first pointed out by Owen ('55, p. 310) 

 and later so elaborately described by the younger Milne- 

 Edwards ('72). 



This relation between the neural artery and the ventral 

 nerve chain is not confined to the Arachnids and Limulus ; a 

 large supra-neural vessel occurs in the Isopods and a smaller 

 one in the Amphipods, each connected with the dorsal vessel 



1 Houssay ('87) claims that this vessel in the scorpion is lacunar rather than 

 arterial, a view which is negatived by its well-marked walls and its lack of connec- 

 tion with the other extra-vascular spaces of the body. 



