PROTOZOA ! INFUSORIA. 



79 



or by a true sexual process. In this latter method two Para- 

 mcetia come together, and adhere closely to one another by 

 their ventral surfaces. The " nucleus," which is truly an ovary, 

 enlarges, and a number of ovules are formed in its interior. In 

 like manner, the " nucleolus " of each, which is really a testis or 

 spcrmarium, also enlarges, and develops in its interior a number 

 of fusiform, or rod-like bodies, which are believed to be sperma- 

 tozoa. The nucleolus of each then passes into the body of 

 the other, the act of transference being effected through the 



Fig. 15. A, Paramcecium, showing the nucleus () and two contractile vesicles (v) ; 

 JB, ParamcEcium bursaria. (after Stein), dividing transversely ; n Nucleus ; it 

 Nucleolus ; v Contractile vesicle. C, Paramcecium aurelia (after Ehrenberg), divid- 

 ing longitudinally. 



mouth. Contact of the two reproductive elements then takes 

 place, and a number of germs are produced, which, after their 

 liberation from the body of the parent, are developed into adult 

 Param&cia. 



Vorticella (fig. 16, c] is a beautiful flower-like Infusorian 

 which is commonly found in fresh water, adhering to the 

 stems of aquatic plants. It consists of a bell-shaped body or 

 " calyx," supported upon the extremity of a slender contractile 

 stem or "pedicle." The other extremity of the pedicle is 

 fixed to some foreign body, and its power of contraction is due 

 to the presence in its interior of a spiral contractile fibre, 

 which is sometimes called the " stem-muscle." The edge of 

 the bell or calyx is surrounded by a projecting rim or border, 

 called the " peristome," within which is a circular surface, the 

 " disc," forming the upper extremity of the so-called " rotatory 

 organ." The disc is surrounded by a fringe of vibratile cilia, 

 forming a spiral line which is prolonged into the commencement 

 of the digestive canal. Near the edge of the disc is situated 



