386 PHYLUM MOLLUSCA. 



cells or chromatophores lying in the connective tissue 

 beneath the epidermis. Each cell is expanded by the 

 contraction of muscular cells which radiate from it, and con- 

 tracts when these relax. It is probable that these chromato- 

 phore cells have some protoplasmic spontaneity of their own, 

 but the controlling muscular elements are also affected by 

 nervous impulses from the central ganglia. As the cells 

 dilate or contract, the pigment is diffused or concentrated, 

 and the colours change. The animal's beauty is further 

 enhanced by numerous " iridocysts " or modified connective 

 tissue cells, with fine markings which cause iridescence. 



Muscular system.- -The cuttlefish is very muscular, 

 notably about the arms, the mantle flap, and the jaws. 

 With great quickness it seizes its prey by throwing out its 

 two long arms, which are often entirely retracted within 

 pouches. With great force it jerks itself backwards by 

 contracting the mantle cavity, and making the water gush 

 out through the pedal funnel. This mode of locomotion is 

 very quaint. At one time the mantle cavity is wide, and 

 you can thrust your fingers into its gape ; when about to 

 contract, this gape is closed by a strange double hook-and- 

 eye arrangement ; contraction occurs, and the water, no 

 longer free to leave as it entered, gushes out by the funnel, 

 the base of which is within the mantle cavity. Another 

 muscular development is interesting, that of the suckers on 

 the arms. They are muscular cups, borne on little stalks 

 (unstalked in Octopus, etc.), well innervated, and able to 

 grip with a tenacity which in giant cuttlefish is dangerous 

 even to men. The inner edge of the cup margin is sup- 

 ported by a chitinoid ring bearing small teeth. Each cup 

 acts as a sucker, in a fashion which has many analogues, 

 for a retractor muscle increases the size of the cavity after 

 the margin has been applied to some object. The external 

 pressure is then greater than that within the cup, and the 

 little teeth keep the attachment from slipping. 



It seems likely that the arms represent a propodium, and 

 the siphon a mesopodium, and a valve within the siphon 

 has been compared to a metapodium. 



Skeletal system. An internal skeleton is represented by 

 supporting cartilaginous plates in various parts of the body, 

 especially (a) in the head, round about the brain, arching 



