236 MORPHOLOGY OF INVERTEBRATE TYPES 



not constitute a connected system. The head contains a cephalic 

 cartilage with several foramina or openings for the various organs 

 such as oesophagus, arteries, nerves, etc., and several smaller 

 cartilages. Other cartilages are situated in the fins, the neck, the 

 siphon, and the mantle. Among them, of special interest are the 

 nuchal cartilage which lies between the liver and the pen, artic- 

 ulating with the latter, and the two infundibular cartilages artic- 

 ulating with the two pallial cartilages. 



Muscular system. The muscular system of the squid is 

 highly developed and complex. The foot and the mantle may be 

 regarded as pre-eminently muscular organs, but muscular layers 

 and specialized muscles are also found in connection with various 

 organs. Excepting the three bands of muscular fibres connecting 

 the mantle with the fins, the thick wall of the mantle is composed 

 of a sheet of circular muscle fibres, covered by and lined with 

 integument. Longitudinal, vertical and transverse fibres form 

 the muscular layer of the fins. In the funnel are both circular 

 and longitudinal fibres. The latter are continuous with the 

 heavy (siphonal) retractors of the funnel which arise one on each 

 side of the funnel and terminate in the pen. A pair of short 

 protractors of the funnel arise from the cephalic cartilage and help 

 to attach the funnel to the head. Inside the funnel, close to its 

 external opening and situated on the upper wall, is a muscular 

 valve preventing the entrance of water into the funnel from the 

 outside. On each side of the funnel is a muscular sheet or valve 

 projecting into the mantle cavity and preventing the passage 

 of water from the mantle cavity to the outside by way of the free 

 space between the collar and the head. A pair of heavy cephalic 

 retractors arise from the cephalic cartilage and terminate in the 

 middle of the pen, just behind the so-called nuchal retractors. The 

 muscular system of the arms and tentacles is too complicated to 

 be considered here. Some of the muscles of the various organs 

 will be mentioned in the course of description of the respective 

 systems. 



Digestive system. The digestive system of the squid 



