144 Introduction to Animal Morphology. 



oesophagus, supported by five tendons attached to the 

 radial pieces, and has a muscular coat. There are 

 small glandular caeca along the cesophageal wall. 

 The intestine is long, once or twice coiled within the 

 somatic cavity, and held in its place by fenestrated 

 mesenteries from the wall of the ciliated perivisceral 

 cavity. Caeca are present in some Clypeastridae, 

 projecting between the pillars of the shell. The end 

 of the intestine (rectum) has firmer mesenteries than 

 the middle. The intestinal wall consists of an outer 

 serous ciliated layer, a middle muscular, and an inner 

 also ciliated with a layer of brown or yellow (hepatic ?) 



Cells. 



The heart* is fusiform, in a common sheath with 

 the stone-canal, having an outer ciliated, a middle 

 spiral or reticular muscular (thick in Cidaris), and an 

 innrr t^s.-llat'd epithelial layer. The circum-oral 

 ring into which thi' heart opens sends radial vessels, 

 to the muscles of the jaws and to one side of the \\ 

 tinal wall. Along the opposite side of the intestine 

 there is a second vessel, the intestinal vein, receiving 

 the blood from the wall of the intestine, and the chyle 

 from the digested food. This vessel breaks up, as it 

 approaches the perisome wall, into branches which 

 ramify on the wall, where their contents become 

 aerated. From hence the dark yellow blood is re- 

 turned by fine branchial vessels to the circum-anal 

 ring from which the heart arises. The heart current 



* Perrier describes the heart as a gland, and denies the existence of 

 any other ring but the water-vascular one. He describes the passage of a 

 vessel from opposite the right anterior Polian sac to the intestine joining 

 the intestinal vein, thus uniting the ambulacral and circulatory systems, a 

 connexion also described as existing in Spatangus and Toxopneustes by 

 Hoffnuut. 



