700 



ZOOLOGY 



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 {st.) ; the 

 lip of the cup is membranous, 

 and immediately within it is a 

 narrow, horny rim (dent.). 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 nar- 

 row, 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 indication of a change termed hecLocotylisation, 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 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. 



Fig. 053. — Sepia cultrata. Entire iinimal 

 viewed from the antero-dorsal aspeet. 



