86 Evolution and Distribution of Fishes 



that nearly always keep a course in close proximity to the 

 lateral nerves, and so are widely apart from each other. 



The position and course of the dorsal vessel exactly 

 agree with what we find in the dorsal aorta of vertebrates. 

 In both also numerous lateral metameric vessels are given 

 off to the body tissues, from near the head on to the tail 

 region. The lateral vessels are of more doubtful value, 

 but seem to have a direct homology with one of two ver- 

 tebrate systems, namely either the lateral veins or the 

 posterior cardinals. But since they are in line with two 

 cephalic veins, and fall into a common "venous sinus" cavity 

 with these, they probably together behave as anterior and 

 posterior cardinals. Now if the three main nerves — the 

 dorsal and the two laterals — stand in intimate relation to 

 these vessels for metabolic purposes, we might expect that 

 when the lateral nerves gradually rose upward to unite 

 dorsally, these cardinals would follow a similar course as 

 transition took place from nemerteans to primitive fishes. 

 Their position in vertebrates suggests that this is what 

 happened, while a transition condition of blood vessels and 

 nerves is seen in Langia, one of the marine heteronemer- 

 teans. Such a migration dorsad of the lateral veins, from 

 an originally lateral position, alongside the lateral nerves, 

 is strong proof that the two plates or lateral thickenings 

 of the spinal cord, represent the dorsal uprising and fusion 

 of two originally lateral nerves. 



Again In several metanemerteans, and for Amphiporus 

 pulcher in particular (6:243) an anterior pair ot vessels 

 forms abundant anastomosing ramifications behind the 

 brain and cerebral organs, also in near position to where 

 an anterior as well as succeeding pairs of gill-clefts might 

 be formed In later evolution, for aeration of the blood. 

 These make up the parastomodeal network and the ves- 

 sels constituting it gradually reunite into what seems to be 

 the homologue of the vertebrate ventral aorta. This, as 

 in vertebrates, joins what may well be called the sinus 

 venosus. 



As to the structure of the blood vessels Dendy has made 

 some valuable observations on the Australian land nemer- 

 tean already referred to. He shows microscopically that 



