79 



value in Cancer. In the swimming crabs, however, where 

 the last thoracic appendages are flattened and oar-like 

 and are utilised as an effective rowing organ, such an 

 arrangement is of no mean importance. 



Muscles of the fore-gut. These are 

 described in the section on the Alimentary Canal. 



The Dor so-ventral muscles are described 

 in the section on Respiration. 



It is of interest to note in passing that in the case of 

 those muscles arising from the carapace, there are definite 

 marks on the outside of the shell corresponding in shape 

 and size to the areas of the origin of these muscles. It 

 has been stated above (section on Integument) that the 

 muscle is not directly attached to the chitinous exo- 

 skeleton, but arises from the basement membrane under- 

 lying the epidermis. How, then, can the marks of the 

 muscle attachments be duplicated on the outer side of the 

 exoskeleton ? 



The most probable explanation is that in those 

 regions where the muscles are attached to the basement 

 membrane the cells of the epidermis are in some way 

 affected by the underlying muscles. In this manner the 

 rate of secretion of the integument may have been slightly 

 reduced in these localised areas, thus producing the marks 

 on the outer side of the carapace. 



Muscles of the Abdomen. 



On account of the third, fourth and fifth somites 

 being fused together, the abdominal muscles of the 

 male abdomen differ from those of the female. 



Female abdomen (PL IV, figs. 32, 33). 



There are two sets of muscles working each somite, 



