Class 4. Cephalopoda. 



319 



sbell is usually a thin, horny, elongate plate, occasionally it is some- 

 what thicker, with a calcareous layer below the horn {Sepia) ; it is 

 completely enclosed in a cavity on the dorsal surface,, an invagina- 

 tion of the outer skin. This so-called "internal shell" 

 corresponds with the external shell of Nautilus, and like it is an 

 epidermal secretion. In the Oetopods the shell is altogether absent. 

 (For the very aberrant shell of Argonauta, see below, p. 323). 



The skin is characterised by its constantly changing colour, due 

 to the presence of stellate pigment cells which can contract and 

 expand (chromatophores) . The so-called ink-sac, a peculiar 

 gland connected with the skin, is usually a pear-shaped bag in 

 which an inky fluid is secreted ; it opens into the mantle-cavity 

 close behind or even into the anus, and when the animal is in danger 

 the fluid can be poured through the funnel, rendering the water 

 black. 



The Cephalopoda possess a true, although feebly-developed, 

 internal skeleton in the form of pieces of cartilage, of 

 which the cartilaginous capsule in the head, surrounding the central 

 nervous system, the auditory organs, and in part the eyes, is the 

 most important. Besides this capsule there are usually several smaller 

 pieces in various regions of the body. 



The nervous system is peculiar in that all the large nerve 

 masses, cerebral, pedal, pleural, and visceral ganglia, are closely 

 collected round the oesophagus, and are directly connected by the 

 shortening of the commissures. The eyes are large, and in many 



Fig. 265. Diagrammatic transverse sections through the eyes of various Cephalopoda. 

 A Nautilus, B, C different Dibranchiates. ep epidermis, h cornea, i iris, I inner, I' outer 

 portion of the lens, n optic nerve, r retina, rf eyelid. — Orig. 



forms attain a high development. The simplest occur in Nautilus 

 (Fig. 265 A), where they are deep, saccular, epidermal invaginations ; 

 the cavity communicates with the exterior by a small opening (the 

 eye belongs to the type figured in Fig. 20, 4, but a lens is w a n t i n g) . 

 In the other Cephalopoda a closed optic vesicle i? developed. 



