16 



sides of the siphon. The groove of each cartilage interlocks with a corresponding 

 ridge on one of the partial cartilages. 



The pallial cartilages are ridged linear plates inserted on the inner surfaces 

 of the two lateral points of the mantle. The siphonal with the pallial cartilages, 

 and the nuchal cartilage with the ventral end of the pen , form three sliding 

 joints which allow the siphon and the neck a large amount of motion and 

 which, nevertheless, hold them in place. Considerable force is required, both 

 in the living and the dead squid , to dislocate these joints. 



The Muscular System. 



The muscular organs of the squid include the mantle , the fins , the siphon , 

 the arms, and the cephalic and nuchal retractors. 



The Mantle. The mantle (text figure 6) is a bulging 

 muscular cone , open ventrally and tapering beyond the middle 

 to a dorsal point. The free ventral edge of the mantle 

 surrounds the neck and is called the collar. The tips of the 

 pen and of the two pallial cartilages form three equidistant 

 points on the collar , dividing it into three arcs and , as the 

 tip of the pen extends farther forward , the two arcs adjacent 

 to it do not extend as far back as the arc between the two 

 pallial cartilages. The former arcs are opposed to the lateral 

 siphonal valves and the latter arc embraces the siphon. 



The mantle is formed of long , coarse , fusiform fibres. 

 Maeceau has shown that each fibre consists of a peripheral 

 sheath formed of somewhat flattened , spirally wound fibrillae 

 separated by sarcoplasm , and a core of granular sarcoplasm 

 which contains the long rod-shaped nucleus. The fibres are 

 all parallel and form circular muscle bands which extend from 

 one side of the pen around the mantle cavity to the opposite 

 side or pass outside the pen and so completely encircle the 

 body. The muscle fibres are bound into rectangular bundles 

 by means of thin muscular septa which bind together the two 

 fasciae that lie on the inner and outer surfaces of the mantle. 

 The mantle is covered and lined by the integument which is 

 reflected from its inner surface over the visceral mass and 

 helps to attach it to the mantle. In addition to this attach- 

 ment and to the articulation of the collar with the body, the mantle is attached 

 to the visceral mass by a pair of ligaments which arise near the middle of the 



