Neiv Forms of the Hemichordata. 171 



gether on the left side (ventral ?) of the alimentary canal to form the 

 efferent vessel. This passes downwards and joins the contractile 

 afferent vessel, by which the blood is again driven through the same 

 circle. Blind diverticula occur in the course of this vascular system, 

 viz., the tentacular vessels, a group of vessels floating freely in the 

 body cavity some distance below these, and finally a number of 

 vessels penetrating the gonads at the other end of the body. 



The intervals between the contraction of the dorsal vessel were 

 very uniform in any one individual under observation, but varied 

 much in different individuals. In some they were three seconds, in 

 others as much as ten. They appeared also to differ in the same 

 individual at different times. 



If the movement of the blood be carefully observed in a 

 favourable position of the animal it is found that after each con- 

 traction of the dorsal vessel is completed there is a very slight 

 return of the blood in its upper part, the quantity returned 

 however being infinitesimal as compared to the large volume passed 

 upwards, and it is only in the upper part of the afferent vessel 

 and carried forward again in the next flow. The same thing happens 

 in the commencement of the efferent vessels. This may be 

 expressed more definitely in the time observations in a particular 

 case. Thus in one case the interval between the contraction of the 

 dorsal vessel was ten seconds. In the afferent vessel the flow of 

 blood continued from the first to the sixth second, in the seventh 

 and eighth there was complete contraction and emptying of the 

 vessel (both of corpuscles and plasma), in the ninth and tenth there 

 was a slight return of blood in the upper part of the vessel. The 

 efferent vessel was never without blood corpuscles. A more decided 

 flow began in the second second and lasted till the eighth, and 

 between such intervals there was a slight return. 



The lumen of the afferent vessel is less than that of the 

 efferent, but the proportions of blood corpuscles to plasma in each 

 seemed the same. The peristaltic contractions of the dorsal vessel 

 were always towards the free or lophophoral end of the animal in 

 normal conditions. 



In injured or regenerating animals there is an oscillatory or to- 

 and-fro movement of the blood in the afferent and efferent vessels, 

 and often a crowding together of blood corpuscles into patches 

 which remain stationary for a time in the free or distal region of 

 the body. 



