THE ANATOMY OF PLEUROTOMARIA BEVRICHII. 239 



which is sometimes regarded as representing the primitive 

 molhiscan stocky viz. the Amphineura, does not exhibit this 

 organ; but, on the other hand, they do not extend back 

 so far in time, the earliest chiton being only found in the 

 Ordovician ; and further, the components of this group, while 

 retaining many primitive features, are obviously specialised 

 along a particular line, so that I do not think the absence of this 

 spiral cEecum in the Amphineura can be regarded as disprov- 

 ing the homology of the two caeca seen respectively in the 

 Cephalopoda and Diotocardia. 



From a consideration, therefore, of the similar structural 

 relations of the spiral ceecum in these two groups, I conclude 

 that the two structures are homologous. 



The intestine (hgs. 7 and 12, int.) is very simple. It runs 

 forward until within about half an inch of the salivary glands, 

 and then forming a U-shaped bend, it passes back towards 

 the stomach, whence it curves dorsally, perforating the 

 pericardium and the ventricle, and bending once more on itself, 

 it enters the mantle-cavity, to the roof of which it is attached, 

 at first slightly to the right of the middle line, but gradually 

 assuming a more median position (figs. 5 and 7, r.). It is 

 attached below the hypobranchial gland, and opens into the 

 mantle-cavity by the anal oritice situated some considerable 

 distance from the posterior limit of the mantle-slit, and there- 

 fore very differently from the condition observed by Dall in 

 P. Adansoniana. 



The Vascular System. — The heart, which is enclosed in 

 a spacious pericardium (figs. 7 and 23 — 26), is that of a 

 typical Zygobranch, consisting of a muscular ventricle (v.) 

 surrounding the rectum, and a pair of thin-walled auricles (l. 

 au. and r. au.), which receive the blood from the long efferent 

 branchial vessels. 



A common aorta springs from the posterior portion of the 

 ventricle, and soon divides into an anterior and a posterior 

 artery ; the former (figs. 6, 7, and 28, a. a.) is distributed to 

 the anterior and ventral parts of the body, while the latter 

 supplies the stomach, liver, and genital gland. 



