98 



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 



per.. 



H' 



FIG. 74. Vorticelln. A, B, living specimens in different positions ; C, optical section ; l)i, 

 D 3 , diagrams illustrating coiling of stalk ; E i , E%, two stages in binary fission ; E"\ fret: 

 zooid ; F l , F 2 , division into mega- and rnicrozooids ; Q l , G 2 , conjugation ; fl 1 , multiple 

 fission of encysted form ; fl 2 , 7? ;i , development of spores ; ax. f. axial fibre ; cort. cortex ; 

 cu. cuticle ; c. vac. contractile vacuole : d. disc ; gnll. 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 



