178 ZOOLOGY FOR MEDICAL STUDENTS chap. 



more especially the burbot {Lota) and perch {Perca). Consequently it 

 occurs as a parasite of man in regions where fresh-water fish form a 

 staple article of diet. 



Tape-worms live in the intestine of the host amongst the digesting food 

 and they nourish themselves by absorbing the products of digestion but 

 although they reach a relatively large size it is difficult to believe that 

 the amount of food which they purloin from the host can in itself be of 

 any appreciable importance. Where actual pathological effects are pro- 

 duced by them, as in the case of the severe anaemia sometimes produced 

 by Bothriocephalus latus, this would appear to be due to the metabolism 

 of the tape-worm producing a toxin which is absorbed into the blood 

 of the host. 



The mode of infection by the various species of Cestoda will have 

 become clear from their life-histories. In general it may be said that in- 

 fection with the larval (bladder-worm) stage is brought about by swallowing 

 the egg, while infection with the adult (tape-worm) stage is brought about 

 by swallowing the scolex contained in the bladder-worm. Consequently 

 the precautions to be taken against infection are in the case of the first 

 mentioned such as are dictated by ordinary cleanliness, in particular the 

 prevention of possibly infected animals from licking the skin or dishes 

 used for food, and in the case of the second care that fish or m'eat is 

 sufficiently cooked to destroy any larval cestodes that may be present in it. 



NEMATODA 



An excellent example of the nematode worms is afforded by the genus 

 Ascaris which is a common parasite in the intestine of the horse and 

 pig- 



Ascaris 



The adult Ascaris (Fig. 8i) is a cylindrical worm tapering towards 

 its two ends and measuring in the female of A. megalocephala, the species 

 found in the horse, as much as 390 mm. in length : the male is smaller, 

 about 180-200 mm. Apart from its smaller size the male is easily dis- 

 tinguished by the tail end of the body being curled in a ventral direction. 

 The surface of the body is covered with a smooth cuticle ; the skin is 

 devoid of pigment ; the body shows no trace of division into somites, 

 and there are no parapodia or other conspicuous projections. 



The body-wall of the Ascaris when examined in microscopic sections 

 is found to possess many interesting features (Fig.' 82). The cuticle (c) 



