82 



Male abdomen. 



There are no extensors between the third and fourth 

 somites, and also between the fourth and fifth somites. 



According to Williamson, there are only two long 

 flexors at each side arising from the thorax. One is 

 inserted on the sternum of the united third, fourth and 

 fifth somites, and the other is inserted on the telson. 



Uropods of Male. These muscles have been described 

 by Williamson.* In the first appendage the endopodite 

 has a strong flexor muscle. The extensor is extremely 

 small, and probably the flexion is effected by the elasticity 

 of the arthrodial membrane. The protopodite has two 

 small muscles, one of which flexes and the other rotates 

 the limb. In the second appendage there is also a strong 

 flexor in the endopodite. The protopodite has a system of 

 small muscles which rotate, extend and flex the appendage. 



Histology of Muscle. 



The muscles of Cancer are composed of striated fibres. 

 Each fibre is an elongated multi-nuclear cell which 

 reveals, in longitudinal sections and in stained prepara- 

 tions, two kinds of striations longitudinal and transverse. 

 As a rule, the cross striations are the more obvious, and 

 produce the " striped " appearance so characteristic of 

 Arthropod muscle fibres. 



Each fibre is composed of numerous longitudinal 

 fibrils, which give rise to the longitudinal striations. In a 

 transverse section across a fibre it is seen that the fibrils 

 have an unequal distribution, and are usually grouped 

 together into polyhedral areas (Cohnheim's areas). The 

 bundle of fibrils constituting a single area is known as a 

 muscle column. The various muscle columns are separated 



* Williamson, H. C. Twenty -second Annual Report of the 

 Fishery Board for Scotland, p. 104. 



