214 Dr Gairdner's Jnali/s'ts of 



the mouth. Its position varies exceedingly ; in the greater 

 number, such as the Hydatina senta. Rotifer vulgaris, and 

 Eosphora najas, it opens towards the posterior extremity of the 

 animal ; in the first of these it is on the back. In the Kolpoda 

 cucullus it opens into the concave surface of the animal, close to 

 the mouth, from which it is only separated by a tongue-shaped 

 eminence. In some of the spirally pedunculated vorticellae, its 

 disposition is very singular, opening along with the mouth into a 

 common fissure, which is not situate in the centre of the circular 

 ranges of ciliae which surround the anterior extremity of the 

 body, but towards the margin, between two of these concentric 

 circles. 



The mouth merits the notice of the systematologist, from the 

 very precise characters which he can draw from thence for his 

 subordinate divisions. This organ reaches its greatest compli- 

 cation in the Hydatina senta, where it consists of an orifice 

 opening in the centre of a globular head, and provided with 

 a pair of serrated mandibules, each resembling somewhat the 

 single mandibules of some of the moUusca, such as the com- 

 mon Helix pomatia, or those of the echini. When the animal 

 is in the act of taking its food, these mandibules are in perpetual 

 motion, opening and shutting with great rapidity, to absorb the 

 colouring particles brought within their reach by the currents 

 excited by the motions of the ciiije. This very singular organi- 

 zation is certainly one of the most curious phenomena visible in 

 their whole structure, and is perhaps one of the most important, 

 as shewing so close an approximation to animals far removed 

 from them in the zoological series. Each mandibule in the spe- 

 cies which I examined, possessed five distinct teeth, but the 

 number varies from two, three, as far as six. Dr Ehrenberg 

 has since succeeded in demonstrating their real nature, by the 

 use of very fine folife of mica (the whole animal is not more 

 than one-eighth of a hne in length), and has come to the con- 

 clusion that they are separate, simple, hard bodies, enveloped 

 with a fleshy covering, which are ingrained into one another like 

 the fingers of the hands when joined. 



The mouth of the other infusoria is a simple unarmed open- 

 ing, surrounded more or less closely with a greater or less num- 

 ber of ciliae. Its position generally determines their anterior 

 1 



