42 IXVEKTEBRATE ANIMALS. 



kind of frin^'e or circle round the mouth of the calyx. Nearly in the 

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

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

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

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

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

 ordinarily fixed by its thinner cxtreniit}' to some solid object. In 

 Vaginicola (lig. 20, B), again, we have an animalcule chisel}' related 

 to !itentoi\ but having the body protected by a horny or membranous 

 sheath. 



The group of the FlwieJIatrd Iiifusnria is distinguished from that 

 of the Ciliata, by the fact that t/ie body is furiushed icith one or more 

 long lashdike filaments or flagella, lutiaUy "(cithout any cilia. A 

 nucleus and contractile vesicle are present. The flagella of these 

 singular Infnsorians are the m.ain locomotive organs, serving by 

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

 as also to set uji currents by me.'iiis of which food is brought to the 

 oiganism. 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 moutli (e.g., Euglena) ; but most have no distinct 

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

 ciiidal area surrounding the base of the flagellum. This ai'ea is en- 

 closeil by a delicate membrane, which is prolonged upwards round 

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

 Ej. The njovements of the flagellum produce a kind of miniature 

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

 Tlicse " coliai-bearing" Infnsorians po.ssess, as will be subsequently 

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

 bodj' rif a sponge. A niunber of the Flagellated Infusorians j)Ossess 

 a limited numljer of cili.-i, in addition to the flagellum. This is the 

 case, f(jr example, in the curious forms known as Peridininm and 

 (_'er<itiiim (fig. 17, 1)), in which the body is enclosed in a kind of 

 liomy shell with long jirojecting processes. 



In their internal structure, the Flagellated Infu.sorians do not 

 differ essentially from the Ciliated fiirms. They occur both in 

 fi-esh and in salt water, and often f(jrm colonies. They are mostly 

 of very minute size ; and it is difficult tii separate certain forms from 

 certain groups of jjlants [.[Igie). 



