260 HtGfl SCqooL ZOOLOGY. 



surround the mouth — " adoral " — are different from those cloth^ 

 ing the rest of the body ; there is, as it were, a division of labour 

 between the locomotor and the nutritive cilia. A third order 

 (Hypotricha) has the cilia confined to one surface, which there- 

 fore becomes a locomotor surface, and .some of the cilia oii that 

 are wenerall v converted into hooks or spines as in Onychodromus, 

 while the fourth — Peritricha — which contains numerous species 

 tendin<r to V)e attached and to form colonies, have the cilia con- 

 fined to the neighbourhood of the mouth. The Peritricha are, 

 however, by no means motionless ; they generally have a con- 

 tractile fibre in the stalk of such forms as the Bell-animalcule 

 ( Vorticella), but they may be free or parasitic forms swimming, 

 or cree})ing by their adoral cilia, or they may simply be able 

 to retract themselves into a shell or case, in virtue of the 

 contractility of the cell-body. 



23. Many of the Peritricha are found living on the suiface 

 of various aquatic animals, especially in such places as the 

 "ills, where they have the advantage of a continuous change of 

 the surrounding water. Such also is the case in the Suctoria, 

 a group of Infusoria which begin life with cilia, but afterwards 

 settle down and replace them by suckers or tentacles through 

 which, in the absence of a mouth, they take in their nourish- 

 ment. The suckers are tubular, and communicate with the 

 endoplasm, to which they carry the juices of the other Infusoria 

 on which they prey, or of the aquatic animals to which they 

 are attached. Both fresh-water and marine forms belong to 

 this "roup, as well as to the other orders of Ciliata, but the 

 fresh-water forms are naturally best known. 



