THE EARTHWORM 291 



arranged in bundles which have a feathery appearance when 

 cut transversely; and an innermost layer of peritoneum lining 

 the coelome. The setoe, which are seen externally, are embedded 

 in the setigerous sacs, which arise as invaginations of the epidermis 

 and are lined with modified cells of this layer. The movements 

 of the setae are controlled by special muscles. Attached to the 

 inner ends of each pair of setigerous sacs are muscles (Fig. 137 B) 

 which extend anteriorly and posteriorly, and which cause a pro- 

 trusion of the setae by their united contraction, or a slanting in 

 either direction, as the figure shows, by contraction on one. side 

 only. Attached to the two pairs of setae in the side of each segment 

 (Fig. 140) there is a muscle which by its contraction draws the 

 inner ends of the two pairs closer together and thus retracts the 

 setae from their extended position. By the coordinated action of 

 these muscles the setae of any region of the worm can be retracted, 

 thus avoiding friction as the body is extended along the surface or 

 within the burrow; and then protruded with a slant in the proper 

 direction to catch hke minute claws upon the surface as the worm 

 crawls forward or backward. Such a mechanism is well suited for 

 locomotion in a l)urr()w or upon the surface of the ground among 

 grass and other objects, against which not only the ventral but also 

 the lateral setae can find a purchase. 



The ccdome, as has been pointed out in the general description, is 

 divided into a scries of compartments by the transverse parti- 

 tions, or septa, which correspond to the external constrictions 

 between the segments, except toward the posterior end of the 

 animal. Where the digestive tract is modified into its special 

 regions in the anterior end of the worm (Fig. 139), the arrangement 

 of the septa is less regular. In the region of the pharynx, the septa 

 are replaced by muscle fibers which radiate outward to the body 

 wall. There is an opening in the ventral part of the septum above 

 and around the nerve cord, so that the several compartments are 

 not entirely isolated. The ccelomic fluid, which occupies all these 

 cavities between the septa, contains colorless amoeboid cells and 

 might be compared with the lymph in one of the large lymph 

 sinuses in the frog. The coelome, as in a vertebrate animal, is 

 lined throughout with a peritoneum of squamous epithelial cells, 

 except on the surface of the stomach-intestine where the peritoneum 

 is modified as the chloragogue cells, which are columnar. The 

 septa consist of two layers of peritoneum which enclose muscle 



