CESTODA, ASCARIS 



179 



covering the surface is very thick, and it is underlaid by the epidermis or 



ectoderm {ep) which is in the form of a syncytium or plasmodium, a 



continuous sheet of protoplasm containing scattered nuclei. At four 



points in the transverse section — in the dorsal line {d.t), 



the ventral line (z;.Z) and the two lateral lines (L.t) — 



the epidermis is seen to be greatly thickened, so as to 



traverse the whole thickness of the body-wall. Of 



these thickenings of epidermis the dorsal and ventral 



are comparatively narrow, while the two lateral are 



broad. In the latter towards their inner end a round 



opening can be seen (ex) which is the cavity of the 



excretory tube cut in transverse section. This is a 



very remarkable organ, quite unlike the nephridium 



of the annelid. It forms a straight tube consisting of 



a greatly elongated single cell, hollowed out into 



tubular form, and traversing nearly the whole length 



of the body. At its hinder end the tube ends blindly, 



while in front it unites with its fellow to open by a 



minute pore in the mid-ventral line near the front end 



of ihe body (Fig. 81, n). 



Also connected with the epidermis is the nervous 

 system of the Ascaris which is of a comparatively 

 simple character. Anteriorly a nervous ring encircles 

 the alimentary canal and from this there pass back 

 six longitudinal nerve strands embedded in the 

 epidermis. Two of these lie in the substance of the 

 dorsal and ventral line respectively while two others 

 lie on each side, just dorsal and just ventral to the outer 

 end of the lateral line. 



As in parasites generally, there are no eyes or 

 otocysts or other highly developed organs of sense. 



The space between the inwardly projecting shelves 

 of protoplasm forming the dorsal, ventral, and lateral 

 lines is occupied by the muscular system (Fig. 82, m.e) — 

 of special interest in the Nematoda from its consisting 

 of a single layer of large myo-epithelial cells. Each of 

 these cells is widest towards its inner end where it bulges into the body- 

 cavity, and is narrower towards its outer end where it fits in amongst 

 its neighbours. The surface layer of protoplasm in the outer portion 

 of the cell is modified to form the specially contractile substance. The 

 inner end of the cell tapers off into a protoplasmic tail (i), which may 



Fig. 81. 



Female Ascaris as 

 seen from the ventral 

 side, an, Anus ; w, 

 mouth ; n, excretory 

 opening; v.U ventral 

 line : 9 , genital open- 

 ing. 



