MUSCLES OF MOLLUSCA. 343 



the shell (Pecten, Ostrea). Special muscles, which are Interwoven 

 with the integument, serve as retractors of the foot ; these arise from 

 the dorsal portion of the shell, and are sometimes broken up into 

 several pairs. These retractors are also found again in the shelled 

 Gastropoda. They generally form a single, but sometimes a double 

 muscle, which arises from the base of the shell, and which passes to 

 the anterior regions of the body, increasing in size as it does so. 

 It supplies the foot as well as the head, and the anterior region 

 of the digestive tube (pharynx); while further it gives off special 

 bundles to the other protractile regions, that is, to the tentacles and 

 copulatory organ. The muscle which arises from the columella of the 

 shell, and accompanies it, is known as the columella muscle. In the 

 Heteropoda it has a wide origin in the carinate foot. In the Ptero- 

 poda it spreads out into the fins given off from the foot. In addition 

 to these muscles other bundles are given off to the viscera. 



The muscular system of the Cephalopoda is much more differen- 

 tiated, in correlation with the formation of an internal skeleton. 

 Two powerful retractors are attached to the cephalic cartilage in 

 Nautilus ; these arise from the sides of the shell-chamber occupied by 

 the animal. In those Decapoda that have an internal shell these 

 muscles take their origin from the wall of the outer wall of the shell; 

 and in the Octopoda, from a cartilage found at that spot. Two 

 branches are given off from these two muscles, which pass to the 

 funnel. Another and much larger pair of muscles arises in the neck 

 of the animal, and broadens out towards the ventral sui'face, where 

 they pass into the funnel. The muscles in the mantle are also 

 arranged in separate layers, as are also the fin-muscles. Lastly, 

 there is the greatly-developed muscular system of the arms, which 

 partly arises from the cephalic cartilage, and surrounds a canal 

 which passes along the axis of the arm. 



Nervous System. 



Central Organs and Nerves of the Body. 



§ 2G6. 



This system of organs also has points in which it resembles that 

 of the Vermes. The whole of the central apparatus, that is, is divided 

 into a superior ganglionic mass, which lies above the commencement 

 of the oesophagus, the supra-cesophageal or cerebral ganglia, and a 

 ventral mass which is connected with the other by commissures, 

 and forms the inferior or pedal ganglia. They are both paired. 

 The earliest rudiment of the cerebral ganglia is seen as a differen- 

 tiation of the ectoderm, the form-elements of which grow inwards, 

 and are accompanied by the rudiments of the eye (Gastropoda). 

 The relations between the cerebral ganglia, and the higher sensory 



