XII 



PHYLUM MOLLUSCA 



739 



or antero-dorsal and posterior or postero-ventral, its borders right 

 and left. The anterior surface is to be distinguished by its darker 

 colour, and by the firmness of the body-wall, due to the presence 

 in this position of a hard internal shell. 



The aperture of the mouth is surrounded by the bases of the 

 ten arms. These are in pairs, situated to the right and left of the 

 median plane. All of them, with 

 the exception of the fourth pair 

 (the most anteriorly situated pair 

 being reckoned as the first), are 

 stout at the base and taper towards 

 the extremity. When extended 

 they are about two-thirds of the 

 length of the body. The outer 

 surface of each (i.e. that turned 

 away from the mouth) is strongly 

 convex, the inner flat, and beset 

 throughout its length with a 

 number of suckers, which are 

 arranged in four longitudinal rows. 

 Each sucker is in the form of a 

 shallow cup, supported on a short, 

 thick stalk ; the lip of the cup 

 is membranous, and immediately 

 within it is a narrow, horny 

 rim. Into the floor and walls of 

 the cup are inserted numerous 

 muscular fibres. When the sucker 

 is being brought into use it is 

 firmly applied to the surface of 

 the object ; by the contraction 

 of the muscular fibres the cavity 

 of the cup is then enlarged, and 

 a partial vacuum is formed, the 

 result being firm adhesion, owing 

 to the pressure of the surrounding 

 water. The fourth pair of arms, 

 usually known as the tentacles, are 

 comparatively long and narrow, 

 and provided with suckers only 

 towards their free ends, which are somewhat thickened and club- 

 like. In the male the fifth arm on the left side presents a slight 

 modification, some of the suckers being absent. This is an in- 

 dication of a change termed hectocotylisation, which, as will be 

 pointed out in the general account of the class, assumes in some 

 cases a very remarkable character. As the nerves which supply 

 them are derived from the pedal ganglia, there is no doubt that 



\ 



FIG. 662. Sepia cultrata. Entire 

 animal viewed from the antero-dorsal 

 aspect. 



