OF ANIMALCULES. 103 



GENUS XI. 

 CERCARIA. 



An ^' invisible pellucid animalcule^ with a tail." If we con- 

 sider the internal organization of this genus^ it compre- 

 hends a very wide range ; indeed, the different species 

 vary so much that it is difficult to give a general defini- 

 tion of their characters with any degree of accuracy. 



177' Cerc ARi A inquieta. The agitated Cercaria, — The 

 body of this animalcule is capable of being contracted 

 into a globular mass, and at other times of being elon- 

 gated (as shewn in fig. 103), when two small dots, or eye- 

 like appearances, may be discerned ; occasionally there is 

 seen a large circular marking near the tail, which is 

 in constant motion, vibrating and inflecting itself in va- 

 rious ways. It is found in salt-water, and is just visible 

 with the naked eye. 



178. Cercaria gyrinus. The whirling Cercaria. — 

 This animalcule is very small, of a white colour, and glo- 

 bular form, with a flexible tail, which is in rapid motion. 

 In swimming it resembles the tadpole. It is very similar 

 to the C. inquieta, but smaller. Found in infusions 

 of animal substances. 



179. Cercaria gibba. The hunch-back Cercaria. — 



