CEPHALOPODS. 327 



equivalent of the shell of the nautilus, or of the gastropods. It is not attached 

 to the animal by any special muscle, but held to the body by two of the arms, 

 especially developed for this purpose. It is, in fact, merely a receptacle for the 

 ova, but at the same 'time affords protection to the argonaut herself. All the rest 

 of the octopods are without shells of any description. 



The body of a cephalopod consists of a muscular sac, in the cavity of which 

 the viscera are placed. In front of the body projects the head, which, in one of 

 the two main sections into which the class has been divided — namely, the 

 Dibranchiata, or those provided with only one pair of gills — is crowned with eight 

 or ten fleshy muscular arms, in the midst of which the mouth is situated. This is 

 armed with two strong jaws, in shape very similar 

 to the beak of a parrot. They are curved, 

 pointed, and of a horny substance in the two- 

 gilled cephalopods, and somewhat calcareous in 

 the four-gilled group. Within the mouth is the 

 rasping tongue, covered with the sharp siliceous 



. - .", . , -. . , UPPER (a) AND LOWER (b) PORTIONS OP THE 



hook-like cusps or teeth, arranged in regular BEAK 0P A CUTTLE . FISH (Sepia). 



transverse series, one behind the other. The 



eyes, two in number, are placed on each side of the head, and are of enormous size 

 in some of the decapods. On the ventral side the muscular sac is disconnected with 

 the head, leaving a more or less wide opening admitting the water to the gills. The 

 water is then expelled through the so-called funnel with more or less force, according 

 to the requirements of the animal. If it be at rest, the expulsion of water is carried 

 on very quietly, but, on the contrary, with much greater force if the animal is in 

 motion. Besides water, other secretions from the body are extruded through the 

 funnel, and especially a dark fluid secreted in a special ink-pouch. When disturbed 

 or irritated, this ink is discharged by all cephalopods, excepting the nautilus, and is 

 supposed to be a means of defence. Mixing with and clouding the surrounding 

 water, we can well suppose that the attack of a pursuing fish might be checked, and 

 the squid or cuttle effect its escape in the darkened fluid. The arms, or feet, of the 

 octopods and decapods are more or less elongate and capable of movement in any 

 direction, and are furnished on one side with numerous suckers, by means of which 

 the animal holds on to anything that it may seize with such tenacity that the suckers 

 themselves are liable to be torn away rather than loose their hold. They are often 

 furnished at the edge with a toothed horny or calcareous ring, and connected with 

 the arms by slender stalks. Cephalopods employ their arms in walking and 

 climbing, and, owing to their position, have to progress head downwards when 

 creeping on the sea-bottom. They are connected at the base by a skin, in some 

 species extending some distance up the arms, and forming a sort of umbrella, which 

 is doubtless of use in the capture of their prey. In the nautilus the arms are 

 different, being short, pointed tentacles, unprovided with suckers. 



The nervous system is more developed than that of other molluscs ; it is con- 

 centrated around the gullet, and protected by a cartilaginous plate, a sort of rudi- 

 mentary skull. The skin of the naked cephalopods is more or less thickly studded 

 with points or dots of various colours. These pigment-cells are subject to alter their 

 tint at the will of the animal, which, chameleon-like, assumes very different aspects 



