PROTOZOA. 1 3 



When it is completely expanded the stalk is straight, or slightly 

 curved, while the distal end, or disc, forms a flattish projection 

 bordered by a thickened rim or peristome. Within the peristome 

 is a groove which surrounds the disc and forms on one side a 

 deep depression, the vestibule. In the groove is a row of delicate, 

 hair-like, protoplasmic processes (cilia). The retracted state is 

 initiated by the withdrawal of the disc ; then follows the folding- 

 in of the peristome, so that the body becomes rounded, and, at 

 the same time, the stalk is thrown into a tightly-coiled spiral. 



Smaller free-swimming, stalkless individuals are also met with, 

 simpler in structure and possessing a circlet of cilia near the 

 arboral .end of the body. 



2. Structure. The body is transparent as in Amoeba, and its 

 internal parts can therefore be readily studied. Covering the 

 whole external surface, cilia excepted, is a thin elastic membrane, 

 the cuticle, which may exhibit transverse striations. It is thinnest 

 on the disc and peristome, and thickest on the stalk, of which it 

 forms the sheath. The cuticle is secreted by the underlying pro- 

 toplasm, and owing to its presence the body possesses a constant 

 form, while the protrusion of lobe-like pseudopodia is rendered 

 impossible. The cilia, which may be compared to permanent 

 thread-shaped pseudopodia, pass through holes in the cuticle. 

 The protoplasm is divided into a firm, finely granular, external 

 ectosarc or cortical layer, and a semi-fluid endosarc. The deeper 

 part of the ectosarc exhibits more or less distinct longitudinal 

 striations (myophan striations), probably due to a fluting of the 

 internal surface. This deeper part is produced into a contractile 

 filament, which is slightly twisted in a spiral manner, and traverses 

 the cavity of the hollow stalk, attached here and there to the firm 

 sheath. 



As in Amoeba, a nucleus and contractile vacuole are present. 

 The former is a horse-shoe-shaped band placed in the broad end 

 of the body, just within the ectosarc ; the latter is spherical, and 

 situated nearly the vestibule. There is a small round pamnuchus 

 near the nucleus. 



A rudimentary digestive apparatus can here be distinguished 

 for the first time. The vestibule, which performs the function 

 of a mouth, is continued into a short tube, the pharynx, which is 

 lined by a continuation of the cuticle, and is provided with 

 numerous short cilia. It passes down into the body with a some- 



