298 



ZOOLOGY 



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

 are called the dorsal (Fig. 237, d.I.) and ventrcd (v.l.) 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 lij^s, one median and dorsal (d. Ip), the other two ventro-lateral 

 {v. Ip). A very minute aperture on the ventral side, and about 2 

 mm. from the anterior end, is the cecretory 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 setoi (pn. s.). 

 In the female the reproductive aperture or gonopore is separate 

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

 ihird of the length of the body from the anterior end (Fig. 240, 

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



ex.Jb 



Fio. 23Y.— Ascarls lumbricoides. A, anterior end from above ; B. the same from below ; 

 / C, posterior end of female, D, of male, side view, an. anus ; d. l-p. dorsal lip ; d. I. dorsal line ; 



e.r. p. excretory pore ; p. papilla; ; pn. s. penial seta; ; v. I. ventral line ; v. Ip. ventral lip. 



(After Leuckart.) 



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

 is sharply curved downwards (Fig. 237, B), while in the female (G) 

 its ventral contour is nearly straight. 



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

 delicate, transparent, elastic membrane, of a firm material of 

 albuminoid composition, the cuticle (Fig. 238, cu.). It is divisible 

 into several laj^ers, and is wrinkled transversely, so as to give the 

 animal a segmented appearance. Beneath the cuticle is a proto- 

 plasmic layer (der. epthm.) containing scattered nuclei and longitu- 

 dinal fibres, and representing a, syncytial ectoderm — i.e., an ectoderm 

 is which the cell-boundaries are not differentiated, and whose 

 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.), 

 arranged longitudinally, and bounding the body-cavity. The 

 structure of the muscles is very peculiar : each (Fig. 239, A) has 



