740 



ZOOLOGY 



SECT. 



the arms of Sepia represent a portion of the foot of other Molluscs ; 

 but there is some doubt as to whether they correspond to the fore- 

 foot or to the epipodia of the Gastropoda. The head-region, 

 comprising as it does the arms (which are the chief part of the 

 foot) and the head proper, is termed the cephalopodium. 



The trunk is covered over by the thick integument of the mantle, 

 which terminates toward the oral end in a ridge round the neck. 

 Anteriorly this ridge projects as a prominent rounded lobe under 

 cover of which the head can be partially retracted. Posteriorly it 

 forms the oral lip of the opening of a large cavity bounded by 

 the mantle the mantle-cavity which extends along the entire 

 posterior face of the body almost to the apex. The wide cleft 

 between the oral edge of the mantle and the posterior surface of 

 the body is not the only aperture leading into the mantle-cavity. 

 On the oral side of this cleft is a large tube the 

 funnel (Fig. 667, inf.) opening on the exterior 

 behind the neck, and internally communicating 

 by a wide aperture with the mantle-cavity. The 

 cleft is capable of being almost completely closed 

 by the apposition of a pair of oval projections 

 (mant. cart.) of the inner surface of the posterior 

 mantle-wall near its oral border, and a pair of 

 concave depressions (inf. cart.) on the opposite 

 (posterior) face of the funnel. The funnel is thus, 

 under ordinary circumstances, the main outlet of 

 the mantle-cavity. As such it not only carries to 

 the exterior the effete water of respiration, the 

 faecal matters from the intestine, and the products 

 of the excretory and reproductive organs, but also 

 takes an important part in locomotion, the most 

 important movements of the Cuttle-fish by 

 which it darts rapidly through the water in the 

 direction of the aboral pointed end of the body 

 being effected by rhythmical contractions of 

 the muscular walls of the mantle-cavity causing jets of water 

 to be forced in the oral direction through the funnel. The 

 free passage of water inwards through the funnel is prevented 

 by the presence in its interior of a flap-like valve opening outwards. 

 The water required for respiration and in locomotion is thus drawn 

 in, not through the funnel, but through the partially-closed slit- 

 like pallial aperture previously referred to. The funnel seems, 

 from the source of the nerves which supply it, to be, like the arms, 

 a specially modified part of the foot. 



Fringing each lateral margin of the body is a thin muscular 

 fold the fin which is used as a swimming organ. 



The anterior wall of the body exhibits, as already mentioned, a 

 hard and resistant character owing to the presence of the internal 



FIG. 663. Shell 

 Sepia cultrata, 

 posterior view. Re- 

 duced. 



