126 COMPARATIVE ANATOMY CHAP. 



E. Cephalopoda. 



In the Cephalopoda, a cartilaginous endoskeleton is developed. 

 This not only serves for the attachment of various muscles and 

 muscular membranes, but is also a protection for important organs, 

 especially for the central portion of the nervous system and the eyes. 

 Of the different cartilages forming this endoskeleton the only constant 

 one is the cephalic cartilage. 



1. Tetrabranchia (Nautilus). 



Nautilus possesses only the cephalic cartilage. This is shaped 

 somewhat like an X, with thick limbs. The oesophagus runs up 

 between the one pair of limbs, the other pair serving as supports for 

 the funnel and as surfaces of attachment for its muscles. 



The most important of the muscles is the large paired shell 

 muscle, which corresponds with the columellar muscle of other 

 Molluscs. It arises from the cephalic cartilage, and runs on each 

 side into the band (annulus), by which the body of the Nautilus is 

 attached to the inner wall of the body-chamber (cf. Fig. 32, p. 22), and, 

 like the band itself, is attached to the shell. The muscle leaves a 

 deep scar on the shell (the lobate sutural line). From the lateral 

 edges of the cephalic cartilage, especially that portion of it which 

 supports the funnel, a broad muscle-band, the musculus collaris, runs 

 forward on each side embracing the nuchal part of the body. The 

 two unite on the neck to form the muscular nuchal plate. The 

 ventral lower side of the cephalic cartilage serves for the attachment 

 of the musculature of the tentacles. 



2. Dibranchia. 



The cartilaginous skeleton is much more developed than in 



Xautilus, owing perhaps, to some ex- 

 tent, to the atrophy of the shell. Fins, 

 with their supporting cartilages, for 

 example, are developed only in those 

 forms with internal, degenerated 

 shells. 



'^ The cephalic cartilage (Fig. 109) is every - 



Fio. 109. Cephalic cartilage of Sepia, where well developed. It encloses all those 

 1. Central aperture through which the ceso- central portions of the nervous system which 

 phagus passes ; 2, preorbital cartilage ; 3, are crowded round the oesophagus, being in 

 iam er n ^ e f orm o f a I 10 n ow circular capsule traversed 



by the oesophagus. Processes of this cartilage 



assist in supporting the eyes, and in conjunction with independent, preorbital 

 cartilages form a kind of cartilaginous eye socket. A basibrachial cartilage is 

 found at the base of the anterior arms in some Decapoda. We have further to 



