SEPIA. 



277 



officinalis) x J-. 



Respiratory. 



On each side of the head is a large simple eye ; although 



of the simple type the eye is complex in structure. It has 



all the more important parts of the vertebrate 

 Sensory. , ^ , ^ . . ., , 



eye, such as cornea^ lens^ triSy vitreous humour 



and retina^ and is supplied by large optic nerves from the 

 brain. Just behind each eye is a 



ciliated olfactory pit, and near the Fig. 195.— Ventral View 

 brain is a pair of large otocysts. of a Cuttle {Sepia 

 As is to be expected from its free 

 active life, large size and com- 

 plexity of structure, the "cuttle" 

 has sense-organs far in advance of 

 those found in any other Mollusca. 



The mantle fuses on the lower 

 surface to endose a large mantle- 

 cavity which is blind behind but 

 opens widely at the neck. Just in 



front of this open- ' 



ing lies the siphon, 

 a tube which opens by a large 

 funnel behind into the mantle- 

 cavity, and by a small aperture 

 forwards under the head. The 

 hind edges of the siphon are so 

 arranged that water expelled from 



the mantle-cavity passes through the "louth between them and the 

 ,1 • 1 x^ . . •Ill " siphon " immediately behind it. 



the siphon, but water mhaled 



passes in between the edges of the siphon and the mantle. 



By muscular contraction the animal forcibly ejects 

 water from the mantle- cavity through the siphon, and in 

 this manner drives itself backwards through the water. 



If the mantle be cut open along the mid-ventral line and 

 thrown back, the interior of the mantle-cavity is exposed. 

 The two most conspicuous organs are a pair of large feathery 

 ctenidia, consisting of a median axis and lateral branches, 

 l^hey are purely respiratory. In the middle line of the 

 body the rectum may be seen running forwards and ter- 

 minating in the anus. A little further backwards open the 

 paired excretory pores and the unpaired genital pore on the 

 left side. As in the mussel and the snail, the mantle-cavity 

 is evidently a part of the external surface of the body. 



Note the ten arms with suckers, 



