130 REPRODUCTION. 



we find them after a while attaching themselves, by means 

 of their sucker, to the body of the mollusks. When fixed 

 they soon undergo considerable alteration. The tail, which 

 is now useless, falls off, and the animal surrounds itself 

 with a mucous substance, in which it remains nearly motion- 

 less, like the caterpillar on its transformation into 

 the Pupa. If we remove the little animal from 

 its retreat we find it to be no longer a Cercaria, 

 but an intestinal worm called Distoma, having 

 the shape of Fig. 138, with two suckers. The 

 Distoma, therefore, is only a particular state of 

 the Cercaria, or rather the Cercaria is only the Fig. iss. 

 larva of the Distoma. 



340. What now is the origin of the Cercaria ? The fol- 

 lowing are the results of the latest researches on this point. 

 At certain periods of the year, we find in the viscera of the 

 Lymnea (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 projections 

 like limbs (Fig. 139). 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 prac- 

 tised eye easily recognizes as young Cercaria, 

 Fig. 139. the tail and the other characteristic bifurcated 

 organ (a) within it being 

 distinctly visible (Fig. 140). 

 These little embryos increase //^W?^2^ \, 

 in size, distending the worm 

 which contains them, and Fig. 140. 



which seemingly has no other office than to protect and 

 forward the development of the young Cercaria. It is, as 

 it were, their living envelop. On this account, it has been 

 called the nurse. 



