40 



INVERTEBRATE ANIMALS. 



famous Prussian microscopist, as so many distinct stomachs, in conse- 

 quence of which he named the Infusoria the Polygastrica (Gr. polus, 

 many ; and gustn; stomach). The vibrating cilia which clothe the 

 surface of I'aramacicnii serve partly 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 undei'go 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 aperture 

 {anus) placed near the mouth (fig. 18, d). The only other organs pos- 

 sessed by Paraiiiwcinm are the so-called nucleus and nucleolus, and 

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

 in the cortical layer of the body. The nucleus is a little soKd body, 

 composed of an extei'nal membrane, with granular contents, and 

 having the nucleolus (or paranucleus) firmly attached to its exterior 



Fig. 19.— Ciliated Infusoria. A, Paramxciumy showing the nucleus («■) and two 

 contractile vesicles (v). B, Fc^raijiceclum bursarla (after Stein), dividing trans- 

 versely ; n Nucleus ; n' Nucleolus ; v Contractile vesicle. C, Farammclum anrelia 

 (after Ehrenberg), undergoing "conjugation," two individuals being partially 

 united by their ventral faces. 



in the form of a little spherical particle. The nucleus and nucleolus 

 used to be regarded as coiTesponding with the reproductive organs 

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

 foundation. The contractile vesicle has essentially the same struc- 

 ture as in the Amaha. It is a little wall-less space in the proto- 

 plasm, which is filled with a transparent 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 exteiior has been stated to exist in 

 some cases. Not uncommonly two vesicles are present. It is not 



