BLOOD SYSTEM 449 



cells which, in places, are drawn out into strands connected to the pericardial walls. 

 This tissue is mentioned by Liiders (1909, p. 114), and he probably referred to the 

 suspensory cells when he stated: "Auch trifft man vereinzelte Kerne mit faserigen 

 Auslaiifern an, die mit Ganglienzellen grosse Ahnlichkeit haben, besonders mit denen, 

 die man in den Schalennerven antrifft". But since, in his detailed description of the 

 nervous system, he does not mention a shell nerve, and, so far as I am aware, no such 

 nerve has ever been described in the Crustacea, it is difficult to see what he was actually 

 referring to. 



The heart has five apertures, a pair of ostia postero-dorsally, an opening into a 

 median aorta antero-ventrally — the aortic valve, and a pair of openings through the 

 pericardial floor into a parenchymatous tissue surrounding the gut (see later, p. 464), 

 which I call the hepatic valves (Figs. 5, 11). I use this term merely for convenience, 

 and do not imply any special function of this parenchymatous tissue. 



All these apertures are effectively splits in the heart wall, but the constitution of the 

 ostia diff"ers from that of the others. Each ostium, as Claus described, consists of a pair 

 of parallel muscles which run close together from the median plane obliquely back- 

 wards. Their apposed edges project inwards into the heart cavity as in typical Arthropod 

 ostia and so, on contraction of the heart, they act as valves in preventing the escape of 

 blood. 



The remaining apertures consist of gaps in the muscular wall of the heart, the edges 

 of the gaps being fused with the pericardial floor. The actual openings thus consist 

 of splits in the latter, and the splits are bounded by a development of myofibrils. This 

 can be seen very clearly in one of my preparations, where the heart wall around the 

 hepatic valve has been drawn out into a cylindrical tube which terminates round the 

 split. These apertures must function quite difi'erently from the ostia. The latter act 

 as true valves, only allowing the passage of blood into the heart, whereas the former 

 are not real valves but rather taps. By the contraction and relaxation of their muscles 

 the apertures can be closed and opened, but the passage of blood through them is 

 equally possible in either direction. However, I am calling them valves for want of 

 a better word and because, in all probability, blood passes through them only in one 

 direction. 



Wrzesniowski (1879, p. 539) described the hearts of various Amphipods, and states 

 that these terminate anteriorly and posteriorly in a complicated valve consisting of a 

 thin diaphragm containing a median split, and Wilson (1903, p. 689) mentions similar 

 openings occurring with true ostia in the heart of certain species of Branchhira. 



The connection between the heart and the gut parenchyma was described by Liiders 

 in Gigantocypris (1909, p. 115). However, it was undoubtedly these connections that 

 Claus discovered as early as 1891 in a heart dissected out of a Conchoecissa (p. 41). He 

 mentions the fact that they are not true ostia but simply "freien Streifen zwischen 

 benachbarten Gruppen von Muskelziigen " (p. 42). 



Liiders describes the hepatic apertures as aff"erent structures and states that each is 

 provided with a valve allowing the passage of blood into the heart (p. 116). He did 



