GENERAL CHARACTERS OF INFUSORIA. 



453 



mentary membrane is hardened, so as to form a shield that protects 

 the body on one side only, or a ' lorica ' that completely invests it ; 

 and there are other cases in which it is so prolonged and doubled 

 upon itself as to form a sheath resembling the ' cell ' of a Zoophyte, 

 within which the body of the Animalcule lies loosely, being attached 

 only by a stalk at the bottom of the case, and being able either to 

 project itself from the outlet or to retract itself into the interior. 



Fig. 238. 



/ A'-* 



A, Kerona silurus : — a, contractile vesicle ; b, mouth ; c, c, Ani- 

 malcules swallowed by the Kerona, after having themselves 

 ingested particles of indigo. B, Paramecium caudatum : — a, a, 

 contractile vesicles ; 6, mouth. 



The form of the body is usually much more definite than that of 

 Amoeba or Actinophrys ; each species having its characteristic 

 shape, which is only departed from, for the most part, when the 

 Animalcule is subjected to pressure from without, or when its cavity 

 has been distended by the ingestion of any substance above the 

 ordinary size. The body does not seem to possess much contractile 

 power in its own substance, its movements being principally exe- 

 cuted by the instrumentality of locomotive appendages ; one re- 

 markable instance of contractility, however, is presented by the 

 stalk of Vorticella (Fig. 239). The locomotive appendages, which 

 may all be considered as prolongations of the tegumentary layer, 

 are destitute of any more minute organization ; being, in fact, of 

 the nature of cilia, though sometimes of much larger dimensions, 

 and employed in a different manner. The vibration of Ciliary fila- 

 ments, which are either disposed along the entire margin of the 



