ALTERNATE AND EQUITOCAL KEPRODUCTION. 



343 



Fig. 363. 



521. If we watch these worms, which always abound in 

 company with the mollusks mentioned, we find them after a 

 while attaching themselves, by means of their sucker, to the 

 bodies of these animals. When fixed they soon undergo con- 

 siderable alteration. The tail, which was pre- 

 viously employed for locomotion, is now useless, Fig. 362. 

 and falls off, and the animal surrounds itself with 

 a mucous substance, in which it remains nearly 

 motionless, like a caterpillar on its trans- 

 formation into the pupa. If, however, after 

 some time we remove the little animal from its 

 retreat we find it to be no longer a Cercaria, 

 but an intestinal worm called Distoma, with two 

 suckers, having the shape of fig. 362. The 

 Distoma, therefore, is only a particular state of 

 the Cercaria, or rather the Cercaria is only the 

 larva of the Distoma. 



522. What now is the origin of the Cerca- 

 ria ? The following are the results of the latest 

 researches on this point. At certain periods of 

 the year, we find in the viscera of the Lymncea 

 (one of the most common fresh-water mollusks) 

 a quantity of little worms of an elongated form, 

 with a well-marked head, and two posterior pro- 

 jections like limbs (fig. 363). On examining 

 these worms attentively under the microscope 

 we discover that the cavity of their body is 

 filled by a mass of other little worms, which a 

 practised eye easily recognizes as young Cer- 

 cari<z,* the tail and the ther characteristic fur- 

 cated organ (fig. 364, a) being distinctly visible 

 within it. These little embryos increase in 



V 



size, distending the worm containing them, and 

 which seemingly has no other office than to 

 protect and forward the development of the 

 young CercaricB. It is, as it were, their living 

 envelope. On this account, it has been called 

 the nurse. 



523. When they have reached a certain size, the young 

 Cercarice leave the body of the nurse, and move freely in the 

 abdominal cavity of the Lijmncea, or escape from it into the 



Fig. 364. 



