432 



MOLLUSCA. 



Mxi 



is pigmented and contains from without inwards (1) a layer of 

 epithelium,* (2) a layer of obliquely-placed plates called the argentea 

 externa, (3) a layer of muscles, (4) the argentea interna, (5) a carti- 

 laginous capsule (K), which lies next (6) the spreading out fibres of 

 the optic nerve. 



At the side of the optic ganglion within the orbit is a glandular 

 body called the white body ( W). 



Alimentary canal. The mouth, which is placed within the circlet 

 of arms, is surrounded by a circular fold forming a kind of lip (Fig. 



347, L\ and, in addition, in the 

 Dibranchs by a membrane called 

 the buccal membrane. The buccal 

 membrane is in some Decapods 

 divided into lobes alternating with 

 the arms and bearing suckers. The 

 entrance to the mouth is armed 

 with two powerful horny jaws, an 

 upper and a lower (Mxi, Mxs), which 

 resemble in form a reversed parrot's 

 beak, the lower jaw being the larger 

 and overlapping the upper. The 

 radula (Ha), which arises in a 

 sheath, usually has in each row a 

 median tooth and three teeth on 

 each side, and there may be in 

 addition, outside these, some flat, 

 non-toothed 'plates (the radula is 

 absent in Cirroteuthis). There is 

 a sub-radular organ in front of the 

 radula. The salivary glands (Spd) 

 open into the buccal cavity; in 

 Octopoda there are two pairs, of 

 which the ventral is applied against 

 the buccal mass, while the dorsal is 

 dorsal to the skull. The ducts of 

 the latter unite, and pass forwards 

 with the oesophagus. In the Deca- 

 pods the dorsal glands are alone 

 present. 



* Not very distinct in the figure ; it is the inner lining of the space enclosed 

 by the great cornea] sclerotic folds. 



M 



FIG. 347. Digestive apparatus of Sepia 

 (after W. Keferstein). L lip ; Mxi, Mxs 

 lower and upper jaws ; Ra radula ; Eg 

 supra-buccal ganglion ; Spd salivary 

 gland ; Oe oesophagus ; L liver ; Gg bile 

 duct;(7spsplanchnic(stomach)ganglion; 

 M stomach ; M 1 caecum of stomach ; A 

 anus ; Tb ink sac. 



