50 INVERTEBRATE ANIMALS. 
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 solid body, composed of an external membrane, with 
granular contents, and having the nucleolus firmly attached 
to its exterior in the form of a little spherical particle. Both 
appear to be organs of reproduction, the nucleus being an ovary, 
and the nucleolus a spermarium. The names, therefore, of 
nucleus and nucleolus are extremely inappropriate, as they 
lead to confusion with the wholly distinct structures which re- 
ceive these names in an ordinary animal or vegetable cell. The 
contractile vesicle has exactly the same structure as in the 
Ameba, It is simply a little contractile cavity filled with a 
fluid apparently derived from the digestion, and contracting 
and dilating at regular intervals. There is usually only a 
single vesicle present, but there may be two or more. 
Reproduction in Paramecium (fig. 13) may be effected by 
fission—that is to say, by a simple splitting of the body of a 
single individual into two portions, each of which becomes 
a fresh being. The process of fission may commence at the 
surface, or it may begin at the nucleus. In other cases, two 
Paramecia come together and adhere closely to one another. 
The nucleus and nucleolus enlarge, and the nucleolus of each 
is transferred to the other, apparently through the mouth. As 
the result of this, numerous germs are produced, which, after 
their liberation from the body of the parent, are developed into 
fresh individuals. 
Lpistylis, which we have chosen as the type of the fixed /x- 
Jusoria, may be regarded as essentially similar to Paramecium 
in its anatomical structure. In place, however, of a single free- 
swimming organism, we have now a colony of more or less 
closely related beings, the whole assuming a plant-like form, and 
being rooted to some solid object. The colonies of £pzsty/zs may 
not uncommonly be found adhering to the stems of water-plants 
or to the backs of our common water-beetles, and the trained 
eye readily recognises them as a greyish-white down or nap. 
On placing a portion of this under the microscope, we see a 
number of little oval cups or “‘calyces” (fig 12, @) supported 
upon a branched stem. The stems are quite transparent, and 
are not capable of contracting or of shortening their length. 
The cups, however, have the power of contracting so as to 
alter their dimensions. Each cup contains a sarcode-body, es- 
