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THE BIOLOGY OF MARINE ANIMALS 



Onchidella contain siliceous spicules, and pholads possess siliceous gran- 

 ules. The former animals lack a test. 



Cephalopod Skeleton. Primitive cephalopods were provided with a 

 chambered shell, in the last compartment of which the animal lived. 

 Nautilus (Tetrabranchiata) still retains this external shell, but in dibranchi- 

 ate cephalopods the shell is internal or wanting. In Nautilus the shell is 

 spirally coiled and consists of a series of chambers, separated from one 

 another by curved septa (Fig. 15.6). The terminal living chamber is the 

 largest of these, and is occupied by the body of the animal; the other 

 chambers are filled with gas. All the septa are perforated in the middle, 

 and are traversed by a siphuncle, a tubular prolongation of the visceral 



Fig. 15.6. Shells of Cephalopods, Presented as Schematic Half-sections 

 (a) Nautilus; (b) Spirilla; (c) Sepia. (Redrawn from Naef and Chun.) 



hump. As the Nautilus grows it adds to the shell, and on occasion the ani- 

 mal moves forward and shuts off a chamber behind it by the secretion of a 

 new septum. The Nautilus shell consists of two calcareous layers, the outer 

 porcellanous (dull in texture), the inner nacreous; septa are also nacreous. 

 The shell in decapods is wholly internal (Fig. 15.6). In Spirula it takes 

 the form of a loose spiral, which is divided by septa into a series of cham- 

 bers. In Sepia the shell or cuttle-bone is entirely internal and functions as 

 an endoskeleton. The primitive chambered condition is still recognizable 

 in the presence of oblique calcareous partitions. Cuttle-bone is composed 

 of horny matter on which calcareous material is deposited. Additional 

 internal skeletons are present in the form of cephalic and other cartilages. 

 In Loligo the endoskeleton is reduced to a horny pen (126). 



