PHYSIOLOGY OF THE INVERTEBRATA. 197 



tliree first segments of tlie trunk, its posterior part being, 

 moreover, extended to the middle of the succeeding segment, 

 and its anterior extremity slightly projected within the man- 

 dibular segment. It is provided with four pairs of distinctly 

 defined lateral valvular openings, each pair occurring exactly 

 in the middle of the corresponding segment. Here, the 

 heart is connected to the body wall by slender fibres, the 

 intervening parts being slightly instricted, whereby the 

 dorsal as well as the lateral edges of the heart acquire a 

 regular undulated appearance. The heart of Ci/clcsthcria has 

 its greatest width across the anterior part, located within the 

 dorsal prolongation of the cervical division, whence it tapers 

 somewhat posteriorly. Its posterior extremity is abruptly 

 truncated and furnished with a rather wide medial opening, 

 whereas the anterior extremity contracts to a short aorta 

 through which the blood introduced into the heart is 

 expelled. The lateral openings of the heart are each 

 surrounded by delicate concentric muscular fibres, and 

 limited by two distinctly defined valvular lips. Likewise at 

 the posterior extremity of the heart a valvular arrangement 

 would seem to occur, and the origin of the aorta is marked 

 ott' by two narrow lips closing and opening at regular 

 intervals. Of any other distinctly defined blood-vessels, 

 Dr. Sars could not find any trace, the blood circulating 

 simply within the lacunar interstices between the muscles 

 and the connective tissues. In the shell, these lacunar 

 interstices have a very complicated arrangement, forming a 

 richly anastomosing network of what Dr. Sars calls " blood- 

 rooms." Along the dorsal line, however, the presence of a 

 well-defined longitudinal blood-sinus may be readily de- 

 termined. 



The blood of Ci/clcstheria is colourless, and contains 

 numerous small rounded corpuscles, the course of which may 

 be traced with comparative ease, especially in young trans- 

 ])arent specimens. By the contraction of the heart (about 

 1 50 per minute) the blood is expelled exclusively from its 



