VI 



PHYLUM NEMATHELMINTHES 



305 



have here a peculiar form of alternation of generations, distinguished 

 not by the alternation of a sexual with an asexual form (meta- 

 genesis) as in Hydrozoa, but by the alternation of a hermaphrodite 

 with a dioecious form 

 a variety of heterogamy 

 (page 41). 



One of the most 

 terrible parasites of file 

 Man is Trichinella lfH-</e 

 spiralis (Fig. 253), a 

 minute worm, the male 

 (C) a little over 1 mm. 

 (iV m ) m length, the 

 female (B) about 3 mm. 

 (|- in.). In the adult or 

 sexual condition it lives 

 in the intestine of Man, 

 the Pig, and other 

 Mammals. 



The adult females, 

 which are viviparous, 

 leave the cavity of the 

 intestine and bore into 

 its wall, usually reach- 

 ing the interior of one 

 of the lacteal vessels of 

 the lymphatic system. 

 Here they deposit their 

 young (B, e) to the 

 number of as many as a 

 thousand or more at a 

 time. These are carried 

 passively in the stream 

 of lymph, perhaps ulti- 

 mately in the blood- 

 stream, and thus distri- 

 buted throughout the 

 body. Eventually they 

 travel into the system 

 of voluntary muscles, 

 such as those of the 

 limbs, back, tongue, 

 etc. Each worm then 

 penetrates the sarco- 

 lemma of a muscle- 

 fibre and coils itself 



FIG. 253. Trichinella spiralis. A, encysted form, 

 in muscle of host ; B, female ; <7, male. bh. connective- 

 tissue envelope ; cy. cyst ; de. ejaculatory duct ; 

 e. embryos ; /. fat-globules ; h. testis ; m. f. muscle- 

 fibre ; oe. pharynx ; ov. ovary ; wo. gonopore ; zk, 

 perforated cells of intestine. (From Lang's Com- 

 parative Anatomy, after Glaus.) 



spindle-shaped cyst 

 VOL. i. 



up 



in the 



(cy.) is formed 



muscle substance (A) ; a 

 round it, and the muscle 

 x 



