276 



ZOOLOGY 



SECT. 



living Worm, are respectively dorsal and ventral in position, and 

 are called the dorsal (Fig. 222, d.l.) and ventral (v. 1.) lines : the other 

 two are lateral in position, thicker than the former, and brown in 

 colour, and are distinguished as the lateral lines. The mouth is 

 anterior and terminal in position, and is bounded by three lobes, 

 or lips, one median and dorsal (d. lp.), the other two ventro-lateral 

 (v. lp.). A very minute aperture on the ventral side, and about 2 

 mm. from the anterior end, is the excretory pore (ex. p.). At about 

 the same distance from the pointed and down-turned posterior end 

 is a transverse aperture with thickened lips, the anus (an.), which 

 in the male serves also as a reproductive aperture and gives exit 

 to a pair of needle-like chitinoid bodies, the penial setw (pn. s.). 

 In the female the reproductive aperture or gono r pore is separate 

 from the anus, and is situated on the ventral surface about one- 



FIG. 222. Ascaris lumbricoides. A, anterior end from above ; B, the same from below ; 

 C. posterior end of female, D.'of male, side view an. anus ; <>. //>. dorsal lip ; </. 7. dorsal line ; 

 ex. p. excretory pore ; p. papillse ; pn. s. penial seta? ; v. I. ventral line ; v. tp. ventral lip. 

 (After Leuckart.) 



third of the length of the body from the anterior end (Fig. 225, 

 gnp.). The sexes are also distinguished externally by the form of 

 the short tail, or post-anal portion of the body, which in the male 

 is sharply curved downwards (Fig. 222, D), while in the female (C) 

 its ventral contour is nearly straight. 



Body-wall. The outer surface of the body is furnished by a 

 delicate, transparent, elastic membrane, of a chitinoid nature, the 

 cuticle (Fig. 223, cu.). It is divisible into several layers, ancl is 

 wrinkled transversely, so as to give the animal a segmented ap- 

 pearance. Beneath the cuticle is a protoplasmic layer (der. epthm.) 

 containing scattered nuclei and longitudinal fibres, and represent- 

 ing a syncytial ectoderm i.e. an ectoderm in which the cell-bodies 

 are not differentiated, and its cellular nature is recognisable only 

 by the nuclei. The cuticle is, as usual, a secretion of the 

 ectoderm. 



Beneath the ectoderm is a single layer of muscular fibres (m.\ 



