PLATYHELMINTHES 87 



nucleus, arises in the germarium, but the yolk or food-material 

 which serves to nourish the developing embryo has its origin 

 in another gland, the mtellarium. 



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 

 seminahs which traverses a coiled penis. At the base of the 

 latter is a vesicula seminalis. The penis lies in a genital 

 recess, into which the female organs also open ; the opening 

 of this recess lies behind the pharynx. 



The ovary consists of a number of oval foUicles, 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) lie 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 sheUs. The developement of the summer eggs takes 

 place whilst i'n 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, Meso'stoma Hhreiibergii 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 



