80 DEVELOPMENT OF 



SO much impaired, that it may be looked at for a long 

 time, without giving certain signs of being a living 

 creature. In this full-grown state they are covered by 

 a very thick cuticle, appearing as if they were enveloped 

 in a thick layer of inspissated albumen ; and this albu- 

 minous cuticle is a great obstacle to observations, in con- 

 sequence of its elevated spherical surface refracting the 

 rays of light. 



This Distoma, as it is represented in figs. 5 e-Ji, occurs 

 frequently in the liver and generative organs of two of our 

 larger fresh- water snails, Limnaus stagnalis and Planorbis 

 Cornells, and in some snails, in assemblages of 10-50. In 

 this condition it would scarcely be taken for the same 

 animal with that which we have previously been consider- 

 ing as Cercaria armata. At first sight there is visible 

 beneath the thick cuticle only an irregular network of 

 canals, filled with a fluid containing minute globules ; a 

 closer examination shows us, in a sharply defined circle 

 in the centre of the animal, the acetabulum or sucker; 

 in two semicircles turned towards each other, we perceive 

 the wider ends of the digestive cavity ; and in depressions 

 and tubes which pass through the thick integument, 

 we see both the mouth and the anus, the latter being 

 recognizable by the fluid loaded with cells which it emits, 

 and by its situation between the posterior horseshoe- 

 shaped borders, which serve as a sucker. 



It is not diflicult to comprehend in what way the Bis- 

 tomata, after leaving the pupa case, enter the nobler organs 

 of the snail, since we saw them bmied in its skin whilst 

 assuming the pupa state. I have very seldom been for- 

 tunate enough to meet with them in the skin soon after 

 their quitting the pupa state; they were at that time 

 very active in their motions. I have also found them 

 \"' and 1'" under the skin, on their passage inwards, 

 and have no doubt at all that they afterwards chiefly 

 follow the course of the aqueous canals, and thus enter 

 the animal much in the same way as when, in the form 

 of Cercarice, they came out of it. 



