XII 



PHYLUM MOLLUSCA 



70!) 



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 (Fig. 616), 

 which are arranged in four longi- 

 tudinal rows. Each sucker is in 

 the form of a shallow cup. sup- 

 ported on a short, thick stalk (st.). 

 The lip of the cup is membran- 

 ous, and immediately within it 

 is a narrow, horny rim (dent.}. In 

 the middle of the bottom of the 

 cup is a thick plug or piston (pl.\ 

 which is capable of being drawn 

 downwards by the contraction of 

 muscular fibres inserted into it, 

 the effect, when the rim of the 

 sucker is in contact with some 

 solid object, being to create a 

 partial vacuum in the cavity, 

 resulting in firm adhesion, owing 

 to the pressure of the surround- 

 ing water. The fourth pair of 

 arms, usually known as the ten- 

 tacles, are comparatively long 

 and narrow, and provided with 

 suckers only towards their free 

 ends, which are somewhat 

 thickened and clublike. In the 

 male the fifth arm on the left 

 side presents a slight modifica- 

 tion, some of the suckers being 



absent. This is an indication of a change termed hectocotyhsation, 

 which, as will be pointed out in the general account of the class 

 assumes in some cases a very remarkable character. From their 

 nerve-supply the arms of Sepia prove to be the equivalents c 

 the fore-foot of Triton, which here becomes extended to the region 

 of the head, and divided into a series of processes 

 encircling the mouth. 



FlG oio. Sepia cultrata. Entire animal 

 viewed from the dm-sul u 



