168 MOLLUSCA. 



Class VI. CEPHALOPODA. 



Free oceanic mollusks, with a distinct head ; swimming or 

 moving by long arms [modifications of the foot] placed around 

 the mouth ; the arms often provided with suckers [acetabula]. 

 Mouth with two long beak -like jaws, and provided with a radula. 

 Body sacciform. Sexes distinct. 



The most highly organized of the Mollusca ; the blood, however, 

 is colourless, or with a slightly bluish, greenish, or yellowish tint, 

 due to the presence of copper. The heart consists of a ventricle, 

 two or four auricles, and two aortse. Respiration is effected by 

 two or four branchiae placed within the mantle ; the water enter- 

 ing into the branchial cavity is ejected through a peculiar tube or 

 funnel [infundibulum], corresponding to the epipodium of the 

 Pteropoda. The cerebral ganglia are often protected by a car- 

 tilage or rudimentary cranium, at the base of which are the 

 two organs of hearing [otocystsj. The eyes approach the ver- 

 tebrate type ; they include a cornea, sclerotica, crystalline lens, 

 retina, and vitreous and aqueous humours, and they are moved 

 by muscles which arise from the orbital cartilages. In the Nau- 

 tilus the eye is " a mere cup lined by the retina." 



The change of colour noticeable in these animals in a living 

 state is due to small pigment-cells [chromatophores] capable of 

 dilatation, and, when contracted, appearing as so many specks. 



Reproduction is by ova of comparatively large size ; there is 

 no metamorphosis, and there is no lower form of invertebrate life 

 manifested by the embryo at any time during its development ; 

 on the other hand, " no arrested stage of development of any 

 higher animal would produce any thing like a Cephalopod." 

 [Owen.] 



This class, according to Schmarda, includes 218 living and 1780 

 extinct forms ; it is divided into two orders : 



Arms with suckers DIBRANCHIATA. 



Arms without suckers.... .. TETRABRANCHIATA. 



Order I. TETRABRANCHIATA. 



TENTACULIFERA. 



Four branchiae. Arras more than ten, without suckers. Shell 

 external, camerated. No ink-bag. 



