348 INVERTEBRATE MORPHOLOGY. 



for the eyes, which are further covered by a pair of plates 

 which project anteriorly and laterally from the anterior margin 

 of the disk. At the base of the arms a brachial cartilage, 

 sometimes united with the cephalic mass, is found which 

 serves for the origin of the brachial musculature, and further- 

 more a nuchal plate is present lying below the anterior sur- 

 face of the body just behind the head. In connection with 

 the infundibuluni plates and nodules are developed, the most 

 important of which is the infundibular cartilage on the pos- 

 terior (strictly speaking the ventral) surface of the body in 

 the floor of infundibuluni, nodules being found below the de- 

 pressions on the side of the infundibuluui and the corre- 

 sponding elevations of the mantle which have already been 

 described as interlocking during the expulsion of water 

 through the funnel. Finally, it may be mentioned that the 

 centre of each fin is occupied by a cartilaginous plate which 

 serves for the origin of the muscles which move the fin. 



In harmony with the peculiar modification of the foot 

 there is a considerable amount of differentiation of special 

 muscles in the Cephalopods, which pass from cartilage to 

 cartilage or from the shell to the various cartilages. Leaving 

 aside the general musculature of the mantle and of the arms, 

 mention may be made of the three or four strong retractor 

 muscles, which pass from the shell to the cephalic cartilage 

 and are sometimes fused together to form a single strong 

 muscle which serves to retract the head ; the collaris, which 

 runs on either side of the neck from the infundibular cartilage 

 to be inserted into the sides of the nuchal cartilage ; and 

 finally the adductors and depressor of the funnel, which pass 

 respectively from the cephalic cartilage and the shell to be 

 inserted into the infundibular cartilage. Considerable varia- 

 tion is to be found in the arrangement of muscles in various 

 forms, but the typical arrangement may be regarded as being 

 somewhat as described. 



Like the other organs the digestive system presents a con- 

 siderable amount of differentiation. The mouth opens in the 

 centre of the disk which bears the arms or tentaculiferous 

 lobes, and is guarded by two strong chitinous or partly calca- 

 reous (Nautilus] jaws resembling in form the beak of a parrot. 



