166 ON SYSTEMATIC ZOOLOGY. 



others are covered with a hard shell,, and are denominated 

 Lithophytes. 



V. INFUSORIA (Animalcules}. These are extremely 

 minute animals, destitute of tentacula or feelers, and 

 generally not visible to the naked eye : they are mostly 

 found in infusions of various vegetable substances, and, 

 after becoming dry, do not revive upon being replaced 

 in moisture. 



(2 1 9.) The genera of these orders are thus defined : 



I. INTESTINJE. 

 a. Perforated with a lateral pore. 



Lumbricus. Body round, with fleshy wings. 



Siphunculus. Body round ; head with a narrow cylindrical 



proboscis. 

 Fasciolaria. Body depressed ; furnished with a ventral pore. 



b.' Destitute of a lateral pore. 



Gordius. Body entirely filiform. 



Ascaris. Body round ; both extremities attenuated. 



Hirudo. Body truncated at each extremity ; head and tail 



dilated when in motion. 

 Myxine. Body carinated. Mouth furnished with feelers. 



II. MOLLUSCA. 



Actinia. Mouth placed above. Aperture single ; capable of 

 dilation. 



Ascidia. Mouth above. Aperture double ; one terminal, 

 the other beneath. 



Limax. Mouth placed before. Body with a lateral perfo- 

 ration. Feelers four. 



Vent united ivith the lateral pore. 



Aplysia. Mouth and body as in Limax. Feelers four. Vent 



dorsal, posterior. 

 Doris. Mouth and body as in Limax. Feelers two. Vent 



dorsal, posterior. 

 Tethys. Mouth and body as in Limax. Body with two 



small pores on the left side. 

 Holothuria. Mouth placed before, surrounded with fleshy 



tentacula, or feelers. 

 Terebella. Mouth placed before, surrounded with fleshy 



tentacula or feelers ; but the feelers capillary. 



