40 



INVERTEBRATE ANIMALS. 



famotis Prussian microscopist, as so many distinct stoniaclis, in conse- 

 fjuence of wliich lie named the Jnfiimria tlie I'oli/gastrua (Gr. polus, 

 many ; and gaster, stomacli). The vibrating cilia which clothe the 

 surface of I'arameeeium serve jiartly to drive the animal rapidly 

 through the water, and partly to set up currents by means of which 

 food is conveyed to the mouth. All the nutrient particles obtained 

 in this way undergo the circulation in the central sarcode above 

 spoken of, where they are partially or completely digested. The 

 indigestible portions of the food are got rid of by a second apertiire 

 (an?(.s-) placed near the mouth (fig. 18, a). The only other organs pos- 

 sessed by Paramo'eium are the so-called nucleus and nucleolus, and 

 the contractile vesicle (or vesicles), all of which apj)ear to be situated 

 in the coitical layer of the body. The nucleus is a little solid body, 

 comjiosed of an external membrane, with granuhu contents, and 

 having the nucleolus (or iiarauucleus) fiiinly attached to its exterior 



Fig. 19. — Ciliated Infnsfiria. A, Faramdxinm , slmwing tlie micleus (n) and two 

 contractile vesicles (?')- B, Faramffclmii httrsaria (after Stein), dividing trans- 

 versely ; n Nnclens; ?(.' Nucleolus ; r Contractile vesicle. C, J'arinnrcciiimnureUa 

 (after Elirenberj;), nnder^'oing " coujuyatiou," two individuals Ijciiig Jiartially 

 united Ly their ventral faces. 



in the form of a little s]ilierical jiarticlo. The nucleus and nucleolus 

 useil to be regtuded as corresponding with the reproiluctive organs 

 of the higher animals ; but this view is now known to be without 

 foundatiim. The contractile vesicle has essentially the same strvic- 

 ture as in the Auirrluc. It i.5 a little wall-less sjiace in the proto- 

 jihusm, which is tilled with a trans|iarcnt fluid, and contracts and 

 dilates at intervals. Sometimes it assumes a stellate form (fig. 19, 

 A), or irregular radiating canals can be traced proceeding from it ; 

 while a communication with the e.Kterior has been skited to e.\ist in 

 some casus. Not micommonly two vesicles arc [ircsent. It is not 



