OKIGIN OF INTESTINAL WOEMS. 



21 



Most of the sexually developed Trematoda are parasitic upon the 

 higher Vertebrata, the Cercaria being, in fact, nothing else than 

 young sexless Trematoda, whose instinct it is, to pass out from 

 the inferior animals where .they are produced, into the higher 

 forms in which they attain the power of sexual reproduction. 

 Should those Cercaria which are generated in aquatic molluscs, 

 be able to attain their sexual maturity in the intestines of 

 insectivorous birds or mammals alone, they can only reach the 

 latter locality by entering the larvse of aquatic insects, and then 

 becoming encysted in the manner already described. In this 

 condition they remain, until the new animal in which they have 

 established themselves, having undergone its metamorphosis, leaves 

 the water and is swallowed by some insectivorous vertebrate. 



In the act of digestion the body of the insect is destroyed, together 

 with the capsule of the imprisoned Cercaria, which in this manner 

 finds itself transplanted into those new circumstances which are 

 alone fitted to permit of its further change into a sexual Trematode. 



That this instinctive impulse of the Cercaria to encyst them- 

 selves after emigration, is accompanied by a desire to pass into 

 insect larvse, I assured myself by ocular demonstration. I had 

 procured a large number of specimens 

 of Cercaria armata which had emigrated 

 from the common Lymnaus stagnalis, 

 and put them into a watch-glass filled 

 with water, in company with several live 

 Neuropterous larvse (of the families of 

 the Ephemerida and Perlida). I soon 

 observed, with the microscope, that the 

 Cercaria, which at first, flapping their 

 tails, moved freely about in the water, at 

 last betook themselves to the insect larvse, 

 and crept restlessly about them. It was 

 easy to see from their movements that 



the little worms had some object in view. The Cercaria armata, 

 as is well known, is provided with a spine-like weapon, pointing 



Fig. 15. A. A Cercaria armata viewed from the abdominal surface, a. Oral sucker 

 with the frontal spine showing through it. b. Ventral sucker, c. Digestive apparatus. 

 d. Urinary organ, h. Tail whose root plugs up a pit in the hinder end of the hody, in 

 which the urinary organ opens. B. The same Cercaria viewed laterally a, b, d, have the 

 same signification. e. The frontal spine. The alimentary canal is left out in this 

 view. C. The frontal spine of ihisCercaria very much magnified, and viewed from above. 



Fig. 15. 



