42 IXVEETEBRATE ANIMALS. 



kind of fringe or circle round the mouth of the calyx. Nearly in the 

 centre of this ring, or on one side, is placed the apei-ture of the 

 mouth, -which leads by a short gullet straight into the central soft 

 sarcode of the interior of the body. A nucleus and contractile vesicle 

 are also present, so that in the essential points of its anatomy Vorti- 

 cella does not differ from a free-swimming Infusorian such as Para- 

 ■mwcium. Indeed, a transition between the two forms is found in 

 the so-called Trumpet- animalcule or Stentor (fig. 20, A), which can 

 detach itself and swim about at will, at the same time that it is 

 ordinarily fixed by its thinner extremity to some solid object. In 

 Vaginicola (fig. 20, B), again, we have an animalcule closely related 

 to Stentor, but having the body protected by a homy or membranous 

 sheath. 



The group of the Flagellated Infusoria is distinguished from that 

 of the Clliata, by the fact that the body is furnished with one or more 

 long lash-like filaments or Jlagella, visually without any cilia. A 

 nucleus and contractile vesicle are present. The ilagella of these 

 singular Infusorians are the main locomotive organs, serving by 

 their lash-like movements to drive the animal through the water, 

 as also to set up currents by means of which food is brought to the 

 organism. In many instances, the latter is their principal func- 

 tion, as the animal is fixed by a stalk in its adult condition. A 

 few forms have a mouth {e.g., Euglena) ; but most have no distinct 

 oral aperture, the inception of food being limited to a definite dis- 

 coidal area surrounding the base of the flagellum. This area is en- 

 closed by a delicate membrane, which is prolonged upwards round 

 the lower part of the flagellum as a kind of cup or "collar" (fig. 17, 

 E). The movements of the flagellum produce a kind of miniature 

 whirlpool inside the collar, and the animal is thus supplied with food. 

 These " collar-bearing " Infusorians possess, as will be subsequently 

 seen, a striking resemblance to certain of the cells which form the 

 body of a sponge. A number of the Flagellated Infusorians possess 

 a, limited number of cilia, in addition to the flagellum. This is the 

 case, for example, in tlie curious forms known as Peridinium and 

 Ceratitim (fig. 17, D), in which the body is enclosed in a kind of 

 horny shell with long projecting processes. 



In their internal structure, the Flagellated Infusorians do not 

 diflFer essentially from the Ciliated forms. They occur both in 

 fresh and in salt water, and often form colonies. They are mostly 

 of very minute size ; and it is difficult to separate ceftain forms from 

 certain groups of plants {Algce). 



