STRUCTURE OF FLUKE- WORMS. 105 
sitic life, and with somewhat the same relations to Turbella- 
rians as Lernean parasites have to the normal Copepoda, or 
water-fleas. 
There is always one sucker which usually encircles the 
mouth, the other (ventral) sucker varies in position, and 
sometimes there is, as in the externally parasitic Polysto- 
mide (Aspidogaster, Polystomum, etc.), a sucker on each 
side of the mouth-opening. In some forms there are two 
large chitinous hooks in the median line between the hinder 
suckers, of which there may be several. 
The reproductive glands are more or less complicated, and 
are much as in the Turbellarians. The eggs are formed (as 
in Cestodes, Turbellarians, and Rotifers) by two distinct 
glands, a germigene and a vitellogene, the latter forming the 
nutritive mass which envelops the protoplasmic germ or egg 
proper, the entire mass being afterward enveloped by the 
ege-shell. Frequently two or more eggs are enclosed in 
one shell, The species are mostly monecious, the external 
opening of the oviduct and the large intromittent organ 
being contiguous. 
The development of the egg begins by subdivision of the 
nucleus; the nucleolus then divides, and subsequently the 
protoplasmic mass. The yolk, however, remains entirely 
independent of this division, and serves as nourishment for 
the other cells forming the body of the embryo. From EH, 
Van Beneden’s observations it appears that the eggs of the 
lower flukes, as a rule, undergo total segmentation, and the 
young of the Distomee are hatched in an oval ciliated 
“‘trochosphere ’’ form, without eye-specks, as in Distoma 
and Amphistoma ; or, asin the Polystomee, there is no meta- 
morphosis, but development is direct, the embryo passing 
directly into the adult condition. 
It was not known before the publication of Steenstrup’s 
work in 1842 that certain worms called Cercarie were the 
free larval forms of the Distomes. The Cercaria echinata, 
first. described by Siebold, is like a Distomum, except that 
the body is prolonged into a long extensible tail. This tail, 
says Steenstrup, is formed of several membranes or tubes 
placed one within the other, of which the outermost is a 
