126 COMPARATIVE ANATOMY chap. 



E. Cephalopoda. 



In the Cephalopoda, a cax'tilaginous endoskeleton is developed. 

 This not only serves for the attachment of various mnscles 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 nmsculature of the tentacles. 



2. Dibranchia. 



The cartilaginous skeleton is much more developed than in 



Kaiifilus, owing perhaps, to some ex- 



j /^_ ^, \^''^ -N tent, to the atrophy of the shell. Fins, 



- — ~-_;l';- ' •• ~ ^ with their supporting cartilages, for 



V__ example, are developed only in those 



y ~~^'~ ..-- • xjl forms with internal, degenerated 



i /•""'^V't^^V "" y^Z shells. 



- -^-us:--^ [^ rpj^g cephalic cartilage (Fig. 109) is every- 



FiG. lOO.-CepliaUc cartilage of Sepia, where well developed. It encloses all those 



1. Central aperture through whit-h tlie cfiso- central portions of the nervous system which 



phagus passes ; 2. preorbital cartilage ; 3, ^re crowded round the cesophagus, being in 



chamber for the eye; 4, cartilaginous the form of a hollow circular capsule traversed 



auditory capsule. \, . 



by the cesophagus. rrocesses oi 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 Dccapoda. AYe have further to 



