166 ON SYSTEMATIC ZOOLOGY. 
others are covered with a hard shell, and are denominated 
Lithophytes. 
V. Inrusor1A (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. 
(219.) The genera of these orders are thus defined: — 
I. Intestinz. 
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. Motuusca. 
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 with the lateral pore. 
Aplysia. Mouth and body asin 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. 
