SUBEINGBOM PBOTOZOA. 35 



ing minute organisms to the neighborhood of the mouth, the 

 cilia surrounding this opening directing them to the gullet. 



The body-protoplasm is usually very granular in its cen- 

 tral part, and filled with food-vacuoles and products of diges- 

 tion. Pigment-granules are sometimes present and may con- 

 sist of Chlorophyll, as in Stentor, and one or more excretory 

 contractile vacuoles are always present. The nucleus is usu- 

 ally single, though occasionally two are present, and in the 

 genus Opalina, which occurs in the intestine of the Frog, they 

 are numerous in the adult condition. When single the nu- 

 cleus may be very large and either spherical, elongated, 

 horseshoe-shaped as in Vorticella (Fig. 17, c), moniliform as in 

 Stentor (Fig. 17, B), or otherwise shaped. In addition to the 

 nucleus there are one or two minute structures usually to be 

 found in its vicinity which play an important part in repro- 

 duction and are known as micronixlei (Fig. 17, A, n'). Other 

 differentiations of the protoplasm are also found in certain 

 forms, as, for instance, special bands differentiated so as to be 

 specially contractile and therefore corresponding in function 

 to the muscles of the higher animals, and hence termed niyo- 

 phanes. In Vorticella a more striking differentiation of spe- 

 cially contractile protoplasm occurs (Fig. 17, C, my) ; running 

 in an open spiral through the centre of the supporting stalk 

 of this organism is a strong myophane terminating above in 

 the protoplasm of the animal. When the latter is stimulated 

 the myophane contracts, coiling the stalk into a close spiral 

 and withdrawing the animal from the source of irritation. In 

 some of the Holotricha, such as Paramoecium, numerous mi- 

 nute rod-like structures occur imbedded in the protoplasm 

 near the surface of the body (Fig. 17, A, tr). They are appar- 

 ently defensive in function, since when stimulated they sud- 

 denly, as if by an explosive action, become transformed into 

 long threads or needle-like structures projecting beyond the 

 cilia. These trichocysts also occur in some Flagellates. 



2. Order Suctoria. 



The Suctoria lack the active movements of the Ciliata, 

 being destitute in the adult stage of cilia, and many of the 



