28o MOLLUSC A. 



Along the upper surface the mantle -edges meet and 

 completely enclose the shell, which is therefore invisible 

 She! tal externally. If the mantle be slit the shell may 

 be removed. It consists of a long ovate mass of 

 chitin with a calcareous portion on its under surface, thickened 

 posteriorly. Hence only the two outer layers of the typical 

 molluscan shell are represented. 



But, in addition, Sepia has an important internal skeleton 

 of cartilage. This forms a cranium enclosing the brain and 

 the otocysts and bearing a remarkable resemblance to the 

 cranium of a vertebrate. Other cartilages support the fins 

 and the tentacles. 



Fig. 199. — Semi-diagrammatic View of Heart, Gills and 

 Excretory Organs of Sepia Officinalis. 



Anterior Aorta, n 

 Anterior Vein, 

 Nephridiopore. 



Afferent Branchial. 



Excretory Cells. 



Pericardium, 



Branchial Heart. 

 Nephridial Sac. 



Posterior Aorta. Posterior Branchial Vein. 



The coelom is fairly well developed and to a large extent 

 retains its perivisceral or motor function. The anterior 



Coelom portion surrounds the heart and the " branchial " 

 hearts and is usually known as the pericardium, 

 and the posterior part contains the ovary. Two small aper- 

 tures lead from the front end of the coelom into the paired 

 kidneys, and at the hind end a similar opening leads into 

 the oviduct. ( ? ). 



