LACA DN LENIN TES. 87 
nucleus, arises in the germariwm, but the yolk or food-material 
which serves to nourish the developing embryo has its origin 
in another gland, the vitellarvum. 
In Mesostoma the male organs comprise a testis running 
along each side of the body near the surface; it is of a clear 
white colour, and follicular nature (Fig. 59). The testis com- 
municates with a vas deferens, arising by one or two branches 
from the middle of the gland. The vasa deferentia of each 
side unite just behind the pharynx, and form a short ductus 
seminalis which traverses a coiled penis. At the base of the 
latter is a vesicula seminalis. The penis les in a genital 
recess, into which the female organs also open; the opening 
of this recess hes behind the pharynx. 
The ovary consists of a number of oval follicles, grouped 
together on each side of the stomach. The ovary of each side 
opens separately into the uterus near the opening of the latter 
into the atrium genitale. The yolk glands (Fig. 60) he one 
on each side of the body, and their ducts unite into a common 
channel, which opens into the genital recess. The two uteri 
lie between the testes and the ovary, one on each side of the 
body ; they are a pair of tubes which open right and left in 
the genital recess, and when they are full of eggs they are 
very conspicuous objects. Other unpaired glands open into 
the genital recess, one of these is the bursa copulatrix, another 
forms the receptaculum seminis, and the third is the shell 
gland. 
Mesostoma produces two kinds of eggs: summer eggs, with a 
thin, transparent shell, and winter eggs, with thick, brown, 
horny shells. The developement of the summer eggs takes 
place whilst in the uterus. The winter eggs are laid in the 
autumn, and can withstand a considerable amount of desicca- 
tion. Copulation is said to be reciprocal, and fertilisation 
always takes place in the atrium genitale. 
As a rule, Mesostoma Ehrenbergu is found living in still 
or slowly-flowing clear water, especially where rushes and 
sedge grow, and where the bottom is of clay. It swims quietly 
through the water by means of the undulations of its body, or 
glides with its cilia along the stems of water-weeds. It feeds 
on the minute freshwater crustacea, small worms, and insect 
