384 Shell Life 



modified, is internal, though the Nautilus of the 

 order Tetrabranchiata possesses an external shell. 

 They have powerful jaws developed into some 

 semblance to a parrot's mandibles, and acting like 

 these. Within these jaws there is a comparatively 

 small radula, the jaws preforming much of the 

 work falling upon this organ among the Gasteropoda. 

 The teeth are not numerous, and they do not exhibit 

 that great diversity of form and size seen in other 

 groups. This is largely due to the fact that the 

 Cephalopods are all carnivorous ; and the uniformity 

 of food j)roduces similarity of teeth. 



Their bodies are divided — not always distinctly — 

 into two regions, the head and the trunk. The 

 head is encircled by eight so-called " arms," con- 

 nected at their bases by a skin. Attention to the 

 embryonic development of Cuttles shows that these 

 arms are analogous to the " foot " in the Gasteropods. 

 At first the mouth is in advance of them, but 

 ultimately they come to encircle it. Beneath the 

 arms are two large eyes, much more highly 

 specialised than in other mollusks but differing in 

 structure from the eyes of vertebrates. There is 

 a transparent cornea, an anterior optic chamber 

 between it and the lens, an iris, a posterior optic 

 chamber, and a retina. These eyes are serviceable 

 not merely for transmitting light sensations to the 

 nervous system as in the snails, but for actual 

 observation, as may be proved by experiment. 

 Owing to the concentration of the chief nerve 

 ganglia into one unbroken mass, we have in the 

 Cuttles the nearest approach to a brain afforded 

 bv the Mollusca. The inner face of the arms is 



