390 University of California Publications in Zoology. [ y L - 6 



canal opening, a set of sagittal fibres, very delicate, not organized 

 into bundles, running from the outer cuticular muscle layer of 

 the dorsal surface to that of the ventral surface. The fibres 

 insert directly in the cuticula, as do those of the protractors of 

 the spines. 



In addition to these sagittal fibres, six distinct coats of muscle 

 fibres can be distinguished, in the body proper. These may be 

 grouped according to their action, which gives us three sets of 

 opposing muscles, each set a longitudinal opposed to a transverse ; 

 or they may be grouped according to- their position with respect 

 to the other tissues of the body. Such a division gives us two 

 groups, one of which may be again subdivided into two. 



1. Peripheral, including all muscles lying without the vitel- 

 laria. May be divided into : 



(a) Cuticular, including all muscles lying without the sub- 



cuticula or "Matrix-zellen." 

 (6) Intermediate, including all muscles lying between the 



subcuticula and the vitellaria. 



2. Central, muscles lying within the vitellaria. 



The cuticular muscles have been described in connection with 

 the cuticula (p. 385). Immediately beneath the subcuticular cells, 

 frequently invading their territory, is a layer of muscle-bundles 

 directed around the body, at an angle of about 60 to its main 

 axis. Thus in a cross-section we get only broken segments of this 

 layer, not a continuous sheet, as in the deeper-lying transverse 

 muscle layer. These fibres run in two interlacing sets whose 

 directions are at an angle of 60 to each other. These bundles all 

 break through the subcuticula to insert by fine threads on the 

 cuticula (pi. 42, fig. 55). 



Immediately beneath the outer transverse layer above 

 described are found the bundles of fibres running parallel to the 

 longitudinal axis of the body. These are heavier and grouped 

 into more definite bundles than any previously described. The 

 layer is one or two bundles thick and occurs over the whole body, 

 but becomes very thin in the region of the lateral folds. These 

 fibres insert on the cuticular musculature, and with the outer 

 transverse just described produce the transverse ridges so char- 

 acteristic of the worm in its contracted state. Within this laver 



