VASCULAE SYSTEM OF VEEMES. 171 



and the ccoloin ; the only character which points to the existence of 

 one is the nature of the perienteric fluid. 



The essential arrangement of the course of the circulation is deter- 

 mined by the presence of two longitudinal trunks, which correspond 

 to the principal trunks already noted in the Annelides. The ventral 

 one runs along the wall of the body, while the dorsal one is attached 

 to the enteric canal, and accompanies it along its coils and loops. 

 The blood-current has the same direction as in the dorsal and 

 ventral vessels of the Annelides. 



The two vessels are at their simplest in the young stages of the 

 Sipunculidge. They appear to be connected with one another 

 around the mouth, where they also communicate with the cavities of 

 the tentacles. At the posterior end of the body a number of actively 

 contractile cseca are connected with the dorsal vessel. In Sternaspis 

 these caaca have a special significance, for they are divided into two 

 tufts which project to the exterior and serve as branchige. In the 

 Sipunculidaa similar, but internal, appendages are distributed along 

 the whole dorsal vessel. The dorsal vessel is coiled in Sternaspis, 

 Bonellia, and Echiurus. Where the tentacles are absent it is 

 continued into the ventral vessel by a vascular loop which surrounds 

 the mouth, and which is sometimes broken up into finer vessels. 

 Owing to the formation in the Bonellia? of a large proboscis out of 

 the greatly elongated upper lip, the anterior portion of the vascular 

 system is very much increased in length. The dorsal vessel is con- 

 tinued to the end of the proboscis, and divided into two branches, 

 which pass along its edges and again meet in the body below the 

 mouth. In Echiurus this arrangement is wanting, as is the pro- 

 boscis. The ventral vessel formed from the union of the two 

 vascular loops in Echiurus and Sternaspis, passes backwards, giving 

 off numerous lateral branches as it does so. In Bonellia it divides 

 shortly after it becomes a single vessel behind the mouth ; but later 

 on it again becomes single. It gives off visceral vessels in Echiurus 

 and in Bonellia ; these, of which there is a large number in Echiurus, 

 run in the mesentery. In Echiurus the most anterior of these 

 vessels forms a considerable enlargement on the enteron, whence a 

 ventral enteric vessel, and two anastomoses, which embrace the 

 enteric tube, are given off to the dorsal vessel. This is clearly a mode 

 of connection of the dorsal and ventral vessels similar to that which 

 is repeated several times in the segments of an Annelid. In these 

 forms the arrangement is limited to, or greatly developed at, one 

 point. The variations from the Annelid type are caused by the 

 distance of the alimentary canal from the ventral median line, in 

 consequence of which the anastomosis is not paired, but arises as a 

 single vessel from the ventral trunk. In Bonellia further changes 

 are noticeable. The transverse anastomosis to the dorsal trunk 

 running along the intestine is developed, on either side, into a 

 large tube, from which the dorsal vessel appears to arise anteriorly, 

 since its posterior portion is either wanting, or has become very 

 small in comparison with the widened anterior part. In this 



