ALTERNATE AND EQUIVOCAL REPRODUCTICiN. 



161 



fixed they soon undergo considerable alteration. The tail, 

 which was previously employed for locomotion, is now use- 

 less, falls off, and the animal surrounds itself with a iiucous 

 substance, in which it remains nearly motionless, 

 like the caterpillar on its transformation into the 

 Pupa. If, however, after some time, we remove 

 tlie 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 wo suckers. The Distoma, therefore, is 

 only a particular state of the Cercaria, or, rather, 

 the Cercaria is only the larva of the Distoma. 



340. What now is ihe 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 

 Limnea (one of the most common fresh-water mollusks) a 

 quantity of little worms of an elongated form, 

 V ith a well marked head, and two posterior 

 projections like limbs, (Fig. 139.) On examin- 

 ing these worms attentively, under the micro- 

 scope, 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 Cercarise, the tail and the characteristic 

 furcated organ (a) within it being distinctly visible, (Fig. 

 140.) These little embryos 

 increase in size, distending 

 the worm which contains 

 them, and which seemingly 

 liias no other oflice than to 

 protect and forward the de- Fig. 140. 



ve\opment of the young 



Cercaria. It is, as it were, their living envelop, 

 account, i* has been called the nurse. 

 14* 



Fig. 139. 



On this 



