508 /• PERCY MOORE. [Vol. X. 



circular fibres (Figs. 2 and 4) consist of a more or less com- 

 plete tube of a dense fibrillated (having longitudinal striae) 

 protoplasm which bears on the internal side a bulging mass of 

 granular protoplasm, in which the nucleus is imbedded; similar 

 granular protoplasm fills the lumen of the cortical tube. 

 Minute branches of the cortical substance communicate with 

 neighboring fibres, and thus form an irregular muscular reticu- 

 lum, in the meshes of which the groups of epidermal cells are 

 included (Fig. 3). This is well shown in surface views of the 

 skin stained in methylene blue. 



The more powerful longitudinal muscle coat is continuous 

 throughout the length of the body, and becomes, through 

 branching and complex interlacing, much differentiated at both 

 the anterior and posterior ends. In general, the fibres are 

 arranged side by side in a single layer, or incompletely in two 

 layers ; in the latter case those of one layer show a tendency 

 to slip between those of the other. Most of the fibres have a 

 length equal to a complete somite; but they are so arranged 

 that the ends of adjacent fibres do not coincide by a distance 

 equal to the length of a minor annulus; that is, in any somite, 

 alternate fibres, all around the body, extend for the entire 

 length of that somite through both annuli, while the remaining 

 fibres extend from the posterior limit of the major annulus to 

 the anterior limit of the minor annulus next anterior, thus 

 breaking joints and materially increasing the strength and 

 flexibility of the body walls. This arrangement is particu- 

 larly striking in B. philadelphicus, in which the muscle fibres 

 are much larger. Where fibres join end to end there is an 

 irregular jagged interlocking, which looks like the line of 

 a break across the grain of a board, and doubtless is to be 

 explained on the same principles. The fibres have a 

 structure similar to that of the circular muscles, but are ex- 

 ceedingly variable in the relative amount and arrangement of 

 striated and granular substance. Some exhibit a complete 

 cortical layer with radiating longitudinal markings, within 

 which, as a medullary portion, the granular protoplasm is con- 

 fined with the nucleus; others show a large mass of granular 

 protoplasm, with the striated substance variously arranged, 



