NEMATODA SKIN 125 



white, or of the colour of polished ivory ; they may be opaque or 

 semi-transparent, but pigment spots are rarely developed. 



Minute Nematodes abound in moist soil, around the roots of 

 plants, etc., and may easily be detected with the aid of a lens 

 wriggling about amongst the particles of sand and earth. Of 

 the animal parasites perhaps the most familiar is the " round 

 worm " (Ascaris lumlricoides, Figs. 66 and 67), which inhabits 

 the alimentary canal of man; others are common in domes- 

 ticated animals, as A. mystax in the cat and dog, and A. 

 megalocephala in the horse and ox. They are also found living 

 parasitically in plants (Fig. 77), causing the formation of galls 

 and other pathological growths ; Anguillula (Tylenchus) tritici 

 causes in this way considerable damage to corn, and others 

 attack root-crops, cabbages, etc. The " vinegar eel " {Anguillula 

 aceti), which occurs so often in weak vinegar, is another familiar 

 example of this group. 



The Skin. The body of the worm is encased in a relatively 

 thick, transparent, smooth cuticle, which is turned in at the various 

 apertures, and lines the tubes connected with them for a greater or 

 less distance. The cuticle is in some cases raised to form spikes or 

 hooks, and in certain species, e.g. Ascaris mystax and A. transfuga, 

 it is produced into two lateral fins, which are supported by a 

 thickened triradiate rod of specialised cuticle (Fig. 62); these 

 fins, however, do not run far down the body. As a rule the 

 cuticle is quite smooth, but it may be ringed, as in Filaria 

 laticaudata and in F. denticulata ; and the rings may bear back- 

 wardly-projecting teeth. 



The skin of Nematodes consists of three layers (i.) the 

 above-mentioned cuticle, which is presumably secreted by (ii.) the 

 sub-cuticle or epidermis which underlies it ; the latter surrounds 

 in its turn (iii.) the muscular layer. 



The nature of the sub-cuticle is one of the debateable points in 

 the morphology of the Nematoda. No cell outlines have been 

 detected in it, although nuclei are scattered through it; it is in fact 

 a syncytium, or protoplasmic mass in which cell limits cannot be 

 distinguished. Many of the cells forming it have broken down 

 into fibrils, and these form a close meshwork, which is occa- 

 sionally specialised, as, for instance, round the nerve -cords. 

 Along the median dorsal and ventral lines, and along the lateral 

 lines, this tissue is heaped up in such a way as to divide the 



