60 



MOLLUSCA. 



on the stalk of the optic ganglion and they cause a rapid inter- 

 change of blue, red, yellow and dark colours. In addition to the 

 chromatophores, there is a deeper layer of small shining spangles 

 which produce interference colours, and thus give rise to the peculiar 

 iridescence and lustre of the skin. 



The Cephalopoda possess an internal cartilaginous skeleton, 

 which serves for the protection of the nerve centres and sense organs 



and attachment of mus- 

 cles. In the Dibranchiata 

 this skeleton constitutes 

 a cartilaginous 



-Gst 



which encloses the cere- 

 bral ganglia, resophageal 

 ring, and the auditory 

 organ, while its lateral 

 portions are hollowed out 

 and represent the orbits. 

 There are also (Decapods) 

 optic cartilages, a so-called 

 brachial cartilage and 

 dorsal cartilage, various 

 small cartilages for the 

 closure of the mantle 

 cavity, and fin cartilages 

 for the support of the 

 fins. 



Alimentary canal. - 

 The mouth, which is placed 

 within the circle of arms, 

 is surrounded by a cir- 

 cular fold forming a kind 

 of lip (fig. 532). It is 

 armed with two powerful 

 jaws, an upper and a 

 lower, which resemble in form a reversed parrot's beak. The radula, 

 which recalls that of the Heteropoda, bears in each row a tooth-like 

 median plate, and on each side three long hooks, adapted for drawing 

 in the food ; in addition there may also be some flat non-toothed plates. 

 The oesophagus usually receives two pairs of salivary glands, and either 

 has the form of a simple narrow tube, or presents before its junction 

 with the stomach a crop-like dilatation (Octopods, fig. 535 Jn). 



Fro. 533. Nervous system of Sepia officinalis (after 

 Cheron). Cg, cerebral ganglion ; Vg, visceral gang- 

 lion ; Eg, buccal ganglion ; Spy, suprapharyugeal 

 ganglion ; Tg, ganglia of the tentacles ; Gst, stellate 

 ganglion ; Ot, auditory vesicle. 



