19;j7.] natural sciences of Philadelphia. 101 



lateral may shdw a dozen fibres apiece in the anterior region of the 

 pharanx; the number decreases caudad of this point to six or seven 

 fibres in the middle region of the trunk. Each fibre measures some- 

 what less than 1 a in diameter in its thickest part. It was not possible 

 to determine the precise length of the fibres, but it is probably aljout 

 one-third or one-fourth of the length of the entire animal. Scattered 

 along the muscles, apparently at irregular intervals, arc small spherical 

 nuclei (see fig. 4, v.l.m. 2), each containing a conspicuous karyosome 

 and surrounded by a small and somewhat irregular mass of cytoplasm, 

 the whole having much the appearance of a small mesenchyme cell. 

 The cytoplasmic masses are. in transverse sections, seen to surround 

 one or more muscle fibres. This relation is also shown in fig. 13, which 

 shows a portion of one of the muscles of the head. The nuclei together 

 with the smTOunding cytoplasm, on account of their intimate con- 

 nection with the muscle fibres, are evidently the undifferentiated por- 

 tions of the muscle cells. 



The posterior attachment of all of the longitudinal muscles is the 

 same. As these muscles, passing caudad, reach the last segment they 

 leave the hypodermis (fig. 8), cross the body cavity and, reaching the 

 wall of the rectum, follow it to its union with the hypodermis. At this 

 point the muscles are attached to the body wall l)y means of fine 

 fil)rillae, formed by a brushlike subdivision of the muscle fibres. The 

 anterior attachments of the three pairs of muscles are quite different. 

 From the anterior part of the last segment the two muscles of the medio- 

 ventral pair run close together in the ventral midline to the posterior 

 border of the first trunk segment, where they diverge (compare figs. 5 

 and 6, v.l.m. 1) and, traversing this segment, are attached by a brush 

 of fibrillse to the posterior wall of the oesophagus, at the point where 

 the latter joins the ventral hypodermis, that is, to the inner surface of 

 the outer lip. The ventro-lateral muscles, on the other hand, pre- 

 serve the same relative distance from one another throughout the 

 trunk. On reaching the anterior edge of the mouth each of these 

 muscles gives off one or two fibres which pass directly forward and 

 pierce the lateral lobes of the brain, where they divide into minute 

 branches. These penetrate between the hypodermal cells and are 

 attached to the cuticle l^y conical enlargements of the terminal branches. 

 This manner of attachment to the cuticle, common to all of the muscles 

 terminating in the head, is well illustrated in fig. 23, l.m.i. The 

 remainder of the muscle fibres rapidly converge cephalad of the mouth, 

 and meet at the posterior surface of the brain in the midline. Here 

 they break up into their component fibres. Some of these run directly 



