14 



it until, beyond the middle of the vane, practically all the muscle fibres pass 

 above the pen and the dorsal end of the pen is deeply buried in the mantle. 

 The chief muscles of the visceral mass are attached to the inner or lower surface 

 of the pen. 



B. The Endoskeleton. 



The endoskeleton consists of a number of disconnected pieces of cartilage 

 which protect the brain , form articular surfaces , and support muscles. Omitting 

 the small cartilages which will be described in connection with the eye and the 

 odontophore, the skeleton consists of eleven pieces of cartilage, viz. four paired 

 cartilages, the fin, the pallial , the siphon al , and the preorbital cartilages; and 

 three unpaired cartilages , the skull , the nuchal , and the postcephalic cartilages. 



The Skull or Cephalic Cartilage. 



The skull (Plate I Fig. 1) is a very irregular cartilaginous plate, perforated 

 by a large foramen thru which the oesophagus, the nerves of the viscera and 

 the arteries of the head pass. The ventral surface of the plate is hollowed out 

 for the reception of the optic ganglia and the oesophageal nerve-ring. The lower 

 portion of the plate contains the two large statocystic cavities and the two 

 special depressions for the pedal ganglion in front of, and the visceral ganglion 

 above , the statocysts. The cartilage is prolonged into two strong median , 

 ventral processes , one , the cerebral , above the cerebral ganglion , the other , the 

 pedal , below the ganglion of the same name. These two processes, one above 

 and one below the nerve-ring , are connected by a pair of hour-glass shaped 

 ligaments , the lateral ligaments , which separate the front end of the oesophageal 

 nerve-ring from the optic ganglia. Each ligament is attached above along the 

 greater part of one side of the cerebral process and below to one side of the 

 pedal process. The cerebral and pedal processses, the lateral ligaments, and the 

 lateral portion of the skull form , at each side of the skull , a large concavity 

 which faces outward and forward and which lodges the optic ganglion. The 

 dorsal surface of the skull is a vertical arched plate. Its edge is covered by 

 the insertion of the retractor muscles of the head and its central portion by the 

 liver. The ventral surface is very irregular and is largely covered by ganglia 

 but the muscles of the foot are attached to the cerebral and pedal processes, to 

 the lateral ligaments , and to the upper and lower edges of the cartilage. The 

 two statocysts form a pair of low rounded projections from the dorsal surface of 

 the skull. The saucer or cup- shaped concavities for the visceral ganglion above , 

 and for the pedal ganglion in front of the statocystic capsule have been 



