88 Evolution and Distribution of Fishes 



heart, In the nemerteans as in the cyclostomes, the posterior 

 cardinals run directly beneath the sheath or notochord. 



"In many genera these two vessels unite anteriorly Into 

 a common cavity that seems to correspond to the venous 

 sinus In lower vertebrates, while two anterior veins that 

 often form a loop system In the front part of the head 

 and that also unite with the sinus, would equally correspond 

 to the anterior cardinals. A single vessel In the higher ne- 

 merteans, starting posteriorly from the anal commissure, 

 corresponds In origin, course, and position to the dorsal 

 aorta of most vertebrates, and It, along with the anterior 

 cardinals, gives rise to the above mentioned vascular loop- 

 system round the front part of the head, that agrees with 

 the circulus cephallcus of vertebrates. 



"But further. In many Heteronemertlnea, an oral- 

 cervical pair of vessels (Schlundgef ass-system of Burger) 

 starts from a median ventral vein given off from the 

 cardiac ring or commissure, and which agrees In relation 

 with the ventral aorta of vertebrate types. These oral- 

 cervical vessels brainch repeatedly after the manner of 

 afferent branchial arteries, are In direct connection with the 

 — as we shall term them tentatively — anterior cardinals, 

 and ramifying round the cardiac organ and cephalic groove 

 agree well with the Internal jugulars. The anterior ending 

 of the median ventral vein, from which these spring, and 

 the side vessels given off from it In front of the mouth 

 (Burger, p. 254), agree well with the hyoldean sinus and 

 mandibular veins of lower vertebrates. The posterior, 

 fusion again of the oral-cervical vessels with the sinus 

 venosus-like enlargements of the lateral vessels constitutes 

 another agreement. 



"Vessels which Biirger has called the proboscis-tube 

 vessels arise at their anterior end In vascular swellings, and 

 posteriorly join the lateral (cardinal?) veins. These 

 might therefore be the beginnings of the lateral veins of 

 cyclostomes. 



"As regards the transverse vessels the description that 

 has been given by authors for the cyclostomes could accu- 

 rately apply for the nemertean segmental somatic arteries 

 that are regularly supplied to the myotomes from the dor- 



