XITl] SEPIA 315 



and by contraction aids the process ; there is a valve-like projection 

 inside it which prevents water from being driven back into the 

 mantle-cavity (Fig. 146). 



Since water is sucked in gently and ejected forcibly the animal 

 is propelled in the opposite direction, that is. backwards, by the 

 reaction of the stream against the surrounding water. Sepia can 

 however also swim gently forward by wave-like undulations of the 

 two lateral fins. These fins are flaps of skin projecting from the 

 sides of the visceral hump (14, Fig. 143). 



It has been said that the visceral hump is unprotected by an 

 external shell. This is not strictly true. On the anterior surface 

 of the hump there is an oval plate-like shell completely hidden in 

 a sac formed by the meeting over it of upturned flaps of skjn (14, 

 Fig. 146). From its upper surface project an innumerable number 

 of delicate calcareous plates parallel to one another, the spaces 

 between them being filled with gas so as to give lightness. 



In one or two living Cephalopoda, as for instance the Pearly 

 Nautilus {Nautilus pompilius), and in very numerous extinct forms, 

 there was a large tubular external shell which might be straight or 

 coiled, but which always had 'the peculiarity of having a large 

 number of septa or transverse plates djviding up its cavity into 

 chambers, only the last of which contained the visceral hump, the 

 rest being filled with gas (Fig. 149). In Sepia it is supposed that the 

 chambers have become so small and shallow that the last one simply 

 appears as a plate situated on part of the surface of the hump. The 

 other chambers contain gas as in Nautilus. 



Sepia possess two well-formed ctenidia, each consisting of an 

 axis bearing two rows of thin plates. The axis is suspended 

 from the body by a membrane, and the ctenidia project down- 

 wards instead of backwards as in the Lamellibranchiata (7, 

 Fig. 143). 



As in Lamellibranchiata, there are two kidneys, which open by 

 little papillae placed just in front of the bases of the ctenidia 

 (12, Fig. 143). Just inside the papilla is a narrow opening, the 

 lips of which are folded so as to make it appear like a rosette. 

 This is the internal opening of the kidney : it leads into a lateral 

 prolongation of the pericardium, which is the reno-pericardial 

 canal (5, Fig. 144). The kidney has the form of a wide sac and may 

 perhaps be compared to a U-shaped kidney, like that of Unio, in 

 which the two limbs have become merged in one another. 



The wall of the kidney is smooth, except over the course of the 

 large veins which run beneath its upper and inner wall. Here the 



