STRUCTURE OF FLUKE-WORMS. 105 



sitic life, and with somewhat the same relations to Turbella- 

 rians as Lernaean 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- 

 midce (Aspidog aster, 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 

 egg-shell. Frequently two or more eggs are enclosed in 

 one shell. The species are mostly monoecious, 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 E. 

 Van Beneden's observations it appears that the eggs of the 

 lower flakes, as a rule, undergo total segmentation, and the 

 young of the Distomea are hatched in an oval ciliated 

 " trochosphere " form, without eye-specks, as in Distoma 

 and Ampliistoma ; or, as in the Polystomece, 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'a 

 work in 1842 that certain worms called Cercarice were the 

 free larval forms of the Distomes. The Cercaria ecJiinata, 

 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 



