INFUSORIA. 157 



of rounded fibres (PL XI. fig. 13), irregularly netted 

 and interlacing. The surface of a sponge exhibits 

 minute pores and larger pouting orifices ; the former 

 of which admit currents of water, to be discharged at 

 the latter, both being the mouths of continuous chan- 

 nels. The surfaces of the channels are lined with 

 sarcodic matter, which takes the form of ciliated 

 amoebiform bodies, by which the currents of liquid 

 are produced. 



The horny fibres of sponges are strengthened by 

 little siliceous or flinty bodies of various forms (PL XI. 

 fig. 13 a, bj c), which are imbedded in the substance of 

 the fibres or attached to their surface, and form very 

 curious microscopic objects. They are called spicula 

 (spiculum, a dart), being often of a pointed form. In 

 some sponges they are calcareous. 



INFUSOR'IA. The animals contained in this class 

 are usually very minute, being rarely even perceptible 

 to the naked eye, except when existing in very large 

 numbers, so as to render the water milky, green, or 

 red. They are found in all kinds of water, but espe- 

 cially in stagnant pools and in decomposing solutions 

 or infusions of vegetable matters. The true structure 

 of their bodies is a matter of doubt, some authors 

 having considered them as being highly organized, 

 while others have regarded them as consisting of 

 simple cells ; and whether they are correctly referred 

 to the Protozoa must remain at present a matter of 

 doubt. The body is of various forms, as represented 

 in Plate XI. figs, 16-40. In some of them it con- 

 sists of a simple sarcodic mass, evidently without 

 any outer skin, as shown by its ready adhesion and 

 laceration on accidental contact with foreign bodies ; 

 while in others the surface is regularly dotted with 

 little depressions, or with nodules, so as to resemble 

 a definitely organized structure. 



The most striking character of the Infusoria is the 

 presence of vibratile cilia, which are variously ar- 



p 



