434 Fiof. M'lutosh on the 



tlie ninth body-segment each muscle diminishes to an 

 irregulai'ly rounded area situated over the dorsal blood- 

 vessel^ the two muscles in the preparations being about as 

 broad as the constricted intestinal canal of the region. Then 

 a strong series of oblique fibres appears beneath^ and, indeed/ 

 they are circuniscril)cd by muscles all round. This narrow 

 part merges into the greatly increased dorsal longitudinal 

 muscles of the next segment. Thereafter the two large 

 muscles are at first separated by an intermediate hypo- 

 dermic process, which passes downward to the apex of the 

 oblique muscle and the alimentary and vascular systems 

 beneath ; then a simple sulcus is left, from which the 

 fasciculi in transverse section pass off in a pennate manner. 

 The latter arrangement forms a projecting portion which 

 impinges on the septum between the dorsal blood-vessels. 

 The fasciculi of the other parts of each muscle resemble 

 those of the ventral. By-and-by the median pennate region 

 becomes very distinct and regular, though less separated 

 from the general body of the muscle. In the centre is the 

 chitinous septum, which is attached to the plate between 

 the dorsal blood-vessels ; while from each side of the former 

 the fasciculi proceed in a nearly horizontal manner, except 

 at the lower fourth, where they incline downward like a 

 brush, and their fibres become more evidently striated. The 

 pennate arrangement continues toward the posterior end, 

 near which the fasciculi are deepest next the middle line of 

 the dorsum ; but it disappears before the termination of the 

 muscle, a section near the top of the tail presenting a simple 

 ovoid appearance. 



The advent of the mouth ushers in considerable changes, 

 which may be taken in series according to their position in 

 the body-wall from before backward — in transverse section. 

 In the dorsal hypoderm the section of the small longitudinal 

 muscle appears on each side of the middle line, resting on a 

 narrow transverse muscle which spreads out at each end 

 over the vascular trunks to the tentacle. It has in the 

 middle line inferiorly a central projection to which the 

 vertical fibres of the space formerly alluded to are attached. 

 On each side of the median line, beneath the transverse 

 muscle, is a continuation of the nerve-centre. The muscle 

 is intimately associated with the functions of the oral and 

 post-oral regions, attaining great development for a con- 

 siderable distance, and then, as the longitudinal dorsal 

 increase in magnitude, disappearing from the central region. 

 Just over the origin of the nerve-trunks from the cephalic 

 mass, the muscle forms a continuous transverse band, 



