206 COMPARATIVE ANATOMY 



In the closely allied genera Zoothamnium and Carchesium the 

 stems are branched, and each branch bears a bell-shaped 

 animalcule at its extremity, so that there is a colony com- 

 posed of numerous individuals organically connected with one 

 another by means of the branching stems. The bell-shaped 

 expansion will be called the body, the "handle the stalk. 

 The contractile cord traversing the stalk is the contractile 

 filament ; the discoidal plug nearly filling up the orifice of the 

 bell-shaped body is the disc ; the groove formed between the 

 thickened rim and the disc is the peristomial groove ; and the 

 orifice leading into the interior of the body is the vestibule. 



Having settled on these terms, we may proceed to examine 

 the structure of the animal in detail. The first thing to be 

 observed is the distribution of the cilia. They are not 

 scattered over the whole body, as in Paramecium, but they are 

 reduced to two circles or whorls, one placed on the thickened 

 marginal rim, the other on the edge of the disc. In addition, 

 there are cilia in the narrowed tube or gullet into which the 

 vestibule leads, and thefe is an undulating membrane of large 

 size forming a sort of triangular flap, one side of which is 

 attached along the peristome and the entrance to the vestibule. 

 (Fig. 44, A, 2m.) The size and arrangement of the cilia can 

 easily be understood by reference to the accompanying figure. 

 Because of the restriction of the cilia to the vestibular region, 

 Vorticella and its allies are placed in an order Peritriclia. 



The bell-shaped body consists, as in Paramecium, of cuticle, 

 cortical layer, or ectoplasm, and a soft internal endoplasm 

 occupying its cavity. The cuticle, like that of Paramecium, 

 must be regarded as the surface layer of the protoplasm, in 

 which the alveoli, because of their superficial position, assume 

 a particular arrangement. The cuticle is variously striated or 

 otherwise ornamented in different species. In Vorticella 

 monilata, as has already been said, it is covered with closely- 

 set warty prominences. 



The ectoplasm is the layer immediately beneath and con- 

 tinuous with the cuticle. It exhibits the usual alveolar 

 structure of protoplasm, and differs from the ectoplasm of 

 Paramecium in being devoid of trichocysts. But in Vorticella 

 one can recognise structures of which there was no trace in 

 Paramecium namely, a layer of very fine contractile fibres 

 lying immediately beneath the cuticle. These fibres are not 



