xii PHYLUM MOLLUSCA 761 



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 posterior 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. 658, 

 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 ordi- 

 nary 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 re- 

 spiration 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 Jin 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 

 shell (Fig. 654). This is completely enclosed in a sac of the 

 mantle. Like the body itself, it is bilaterally symmetrical. In 

 shape it may be described as leaf-like, with a rounded and 

 comparatively broad oral end, and a narrower aboral end, provided 

 with a sharp, anteriorly-projecting spine. The posterior surface is 

 convex; the anterior convex towards its oral end, but deeply 



