ZOOLOGY 



SECT. 



similar structure found in so many of the Mastigophora. Some 

 (Fig. 72, 4) have bell-like shells, variously ornamented, and in 

 others (Fig. 73, 1) the similarly shaped shell is perforated and 

 resembles the skeleton of some of the Kadiolaria. A chitinoid 

 plate or operculum (Fig. 73, 2, op.) may be fixed to the edge of the 

 peristome, and, when the animal is retracted in its case, accurately 



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FIG. 74. Vorticella. A, B, living specimens in different positions ; (7, optical section ; D 1 , 

 D z , diagrams illustrating coiling of stalk ; E 1 -, E%, two stages in binary fission E'-', free 

 zooid ; FI, F 2 , division into mega- and microzooids ; G 1 , <? 2 , conjugation ; fl 1 , multiple 

 fission of encysted form ; fl 2 , H a , development of spores ; ax. f. axial fibre ; corf, cortex ; 

 cu. cuticle ; c. vac. contractile vacuole : d. disc ; gull, gullet ; m. microzooid ; mth. mouth ; 

 nu. meganucleus ; per. peristome. (From Parker's Biology.) 



closes the mouth of the latter, or a similar operculum (3) is 

 attached to the interior of the tube, and is closed by a contractile 

 thread of protoplasm (m.), which acts as a retractor muscle. 



Compound forms or colonies are common among the Peritricha, 

 rare in the other subdivisions. Many peritrichous forms occur as 

 branched, tree-like colonies, often of great complexity (Fig. 72, 9a ; 

 Fig. 75). The stem of these may be a purely cuticular structure 



