VORTICELLA 235 



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 ; the tube 

 leading into the interior of the body is the cytopharynx, and 

 its opening into the peristomial groove is the cytostome. 



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 cytopharynx, and there 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 cytostome. (Fig. 50, A, un.) The size and arrange- 

 ment of the cilia can easily be understood by reference to the 

 accompanying figure. Because of the restriction of the cilia 

 to the peristomial region, Vorticella and its allies are placed 

 in an order Peritricha. 



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

 cortical layer or ectoplasm, and a soft internal endoplasm. 

 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 



