266 ELEMENTARY BIOLOGY. [CHAP. 



Pigment granules will be found scattered among 

 its fibres. 



e. The longitudinal muscular /ayer, somewhat thicker 

 than d. Each of its several tracts is subdivided 

 into a great number of fasciculi by uniformly 

 arranged radial septa, within which small blood- 

 vessels are carried. The muscular fibres are 

 arranged in close set parallel lamellae, disposed 

 at right angles to the septa. 



f. The peritoneum, a delicate membrane immediately 

 internal to e. 



3. The setce, and their associated structures. When at 

 rest they project inwardly to the level of the perito- 

 neum. Examine in order 



a. The integumentary sheath of the seta. Trace its 

 origin from the epidermis it can be followed to 

 the inner end of the seta. 



b. The cuticular sheath, carried in with a. It can be 

 traced some distance up the side of the seta. 

 (Cf. Sect. B.) 



c. The muscles of the seta. Examine any one pair 

 of setae and note, passing obliquely downwards 

 from their approximated inner ends, the fan-shaped 

 (in section) protractor fibres. They pass into the 

 circular layer of the body- wall. 



Two sets of retractor fibres will be seen arising 

 from each seta at about its middle. By their 

 union there are formed two ribbon-shaped bands 

 which pass right and left of the setae, upwards 

 and inwards, between the longitudinal fibres of the 

 body-wall and the peritoneum. 



