444 Causes and Course of Organic Evolution 



But furtlier, in many Heteronemertinea, an oral-cervical 

 pair of vessels (Schlundgefass-system of Burger) starts from 

 a median ventral vein given off from the cardiac ring or com- 

 missure, and which agrees in relation with the ventral aorta 

 of vertebrate types. These oral-cervical vessels branch re- 

 peatedly after the manner of afferent branchial arteries, are 

 in direct connection with the — as we shall term them tenta- 

 tively — anterior cardinals, and ramifying round the cardiac 

 organ and cephalic groove agree well with the internal jugu- 

 lars. 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 hy- 

 oidean 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 consti- 

 tutes another agreement. 



Vessels w^hich Burger has called the proboscis-tube vessels 

 arise at their anterior end in vascular swellings, and poste- 

 riorly 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 accurately 

 apply for the nemertean segmental somatic arteries, that are 

 regularly supplied to the myotomes from the dorsal aorta. 

 A corresponding series of somatic veins also empty into the 

 cardinals (laterals). 



As to the methods and direction of blood circulation our 

 knowledge is still vague. Burger considers that the blood 

 flows along the dorsal aorta which is the most powerful pul- 

 sating one. It then flows into the transverse vessels, thence 

 into the lateral vessels, from which it is propelled forward. 

 This if fully proved to be correct would agree fairly well with 

 the cyclostome circulation. Interesting also is Bohmig's dis- 

 covery in nemerteans of unicellular valvular swellings that 

 occur at irregular intervals along the interior of the vessels, 

 and there act as valves for the course of the blood flow. 



The nucleated and often pigmented blood corpuscles con- 

 tain true haemoglobin in not a few cases, both of which are 

 characters that lead up to the vertebrates. 



Before leaving the blood vascular system, a feature that 

 has considerably impressed the writer may be here mentioned, 

 whether much morphological value may attach to it or not. 



