202 UNSEGMENTED " WORMS." 



covered by a cuticle, often thick, usually striate, often subject to 

 moulting ; the muscular system consists of elongated muscle - 

 cells arranged longitudinally , and usually leaving two free 

 " lateral lines" From a nerve-ring around the gullet, six 

 nerves go forwards and six backwards. The alimentary canal 

 is usually -well developed, has mouth and anus, and is divided 

 into three regions. Vascular and respiratory systems are 

 unrepresented; the cavity of the body is not c&lomic ; the 

 remarkable excretory system consists of two lateral canals 

 opening anteriorly by a single pore. The sexes are usually 

 separate and the reproductive organs simple ; there is distinct 

 sexual dimorphism. The males have usually copulatory 

 spicules, and sometimes a membranous bursa. The vulva 

 may be anywhere on the ventral surface, often well fonvard. 

 The life history is often intricate. There are many remark- 

 able features such as the sluggish amoeboid spermatozoa, the 

 almost complete absence of cilia and flagella, and the absence 

 of migratory phagocytes. 



Type, Ascaris megalocephala, the Round-worm 

 of the horse 



This round-worm occurs in the small intestine of the 

 horse, while other species similarly infest man, ox, pig, 

 etc. The body is cylindrical in cross-section and tapering 

 at each end. The colour is dead-white, the absence of 

 pigment being very characteristic of Nematodes. Some of 

 the small thread-worms, e.g. Trichostrongylus pergradlif in 

 the caeca of the grouse, are quite transparent and almost 

 invisible when alive. At the anterior end is the mouth, 

 furnished with three lips bearing sense papillae ; the anus is 

 posterior and ventral. The male is smaller than the female, 

 and has a recurved tail furnished with two horny spines and 

 numerous sense papillae. It is usually about seven inches 

 long, while the female may be as much as seventeen. 



(a) Most externally there is a thick chitinoid cuticle, 

 perhaps of protective value. With its presence may be 

 associated the scarcity of cutaneous glands, and the absence 

 of cilia, (b} Beneath this is the sub-cuticula or epidermis, 

 thickened along four longitudinal lines median dorsal, 



