280 MOLLUSCA. 
Along the upper surface the mantle-edges meet and 
completely enclose the shell, which is therefore invisible 
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, Sefza 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. 
Skeletal. 
Fig. 199.—SEMI-DIAGRAMMATIC VIEW OF HEART, GILLS AND 
EXcRETORY ORGANS OF SEPIA OFFICINALIS. 
Anterior Aorta. 
Anterior Vein. 
Ventricle. 
Auricle. 
Nephridiopore. 
Efferent 
Branchial. 
Afferent Branchial. 
Branchial Heart. 
Excretory Cells, 
Nephridial Sac. 
Pericardium. 
Posterior Aorta, Posterior Branchial Vein. 
The ccelom is fairly well developed and to a large extent 
retains its perivisceral or motor function. The anterior 
portion surrounds the heart and the “ branchial” 
hearts and is usually known as the fericardium, 
and the posterior part contains the ovary. Two small aper- 
tures lead from the front end of the ccelom into the paired 
kidneys, and at the hind end a similar opening leads into 
the oviduct. (¢). 
Celom, 
