SECT, iv THE MUSCULATURE 53 



bent position, the only striking alteration being its 

 development of the sharp ridge round the front as a 

 continuation of the lateral edges of the dorsal fold, 

 so the musculature can easily be traced back to that 

 of a typical Annelid transformed, first by the bend- 

 ing of the body, and secondly by the development 

 of the exoskeleton. 



We shall first describe the musculature in a car- 

 nivorous Annelid, and see what transformations it 

 would undergo owing to the bending of the five 

 anterior segments. Fig. 1 1 is a transverse section 

 of such an Annelid. A rather weakly developed 

 circular muscle layer is found immediately under the 

 hypodermis, and under this runs a strongly developed 

 longitudinal muscle layer, the two forming together 

 the dermo-muscular tube. The development of para- 

 podia leads to important modifications, such as the 

 grouping of the longitudinal muscles into four strong 

 bands, two dorsal and two ventral, each being a chain 

 of segmentally arranged muscular bundles marked off 

 by the transverse dissepiments. The circular muscles 

 are also modified, running out laterally, both dorsally 

 and ventrally, into the parapodia. 



At the posterior end of the body where the 

 parapodia are less developed, we might expect that 

 the muscle bands would gradually spread out to 

 form a more and more complete clermo-muscular 

 tube, the dorsal bands eventually uniting with the 

 ventral in the last segments. 



It is not difficult to describe the changes which would 

 naturally take place in this musculature by the fixing 



