172 



COLLEGE ZOOLOGY 



and a thick stratum of longitudinal muscle fibers (m), mesodermal 

 in origin, lining the ccelom. Thickenings of the ectoderm form 

 the dorsal (d.l), ventral (y.v), and lateral (Jat.l) lines. In each 

 of the last-named lies one of the longitudinal excretory tubes 

 (ex.v). The nerve-cords are also embedded in the body- wall. 

 The intestine consists of a single layer of columnar cells, the 

 entoderm, coated both within and without by a thin cuticle. 



der.epthm' 



i.Tit 



lal.u 



actf- 



Fig. 112 b. — Transverse section of Ascaris lumbricoides. cu, cuticle; dl, 

 dorsal line; der.epthm, epidermis; ex.v, excretory tube; int, intestine; lat.l, 

 lateral line; m, muscular layer; ovy, ovary; ut, uterus; v.v, ventral line. 

 (From Parker and Haswell, after Vogt and Yung.) 



The ccelom (see p. 8c,) of Ascaris differs from that of the 

 higher animals in several respects. Typically the ccelom is a 

 cavity in the mesoderm I^ned by an epithelium ; into it the ex- 

 cretory organs open, and from its walls the reproductive cells 

 originate. In Ascaris the uo-called ccelom is lined only by the 

 mesoderm of the body-wall, there being no mesoderm surround- 

 ing the intestine. Furthermore, the excretory organs open to 

 the exterior through the excretory pore, and the reproductive 



