42 ENTOZOA. 



are still witMn tlie egg- shell. Even before the period of their 

 liberation, they exhibit all the essential organs of reproduction and 

 alimentation, and are therefore ready to assume an independent 

 existence the moment they quit the shell. The eggs themselves 

 are oval, with a slight tendency to become pyriform, being a httle 

 compressed laterally at one pole, where the chorion is prolonged 

 into a single filamentary process, which may be aptly termed the 

 " holdfast." Van Beneden suggestively compares a group of them 

 to a "bouquet of vorticells." On quitting the shell the embryonic 

 Udonella at once attaches itself to the Galigus, and in this 

 situation continues to grow until it acquires the adult condition. 

 The entire course of development is thus rendered extremely 

 simple, and we are hereby confronted with the fact of a trematode 

 worm passing through its life-cycle without the remotest indication 

 of those phenomena of alternate generation which so commonly 

 prevail amongst the true distomes,monostomes, and other digenetic 

 fltikes. 



Genera.-^Tristoma, Cuvier; ^= NitzscMa, Yon Baer ; = Gap- 

 sola, Bosc ; = Phylline, Oken ; Epibdella, Blainville ; Udonella, 

 Johnston ; = Amphibothrium, Trey and Leuckart ; JEncotyllabe, 

 Diesing ; Galicotyle, Diesing ; Benedenia, Diesing ; Notocotyle, 

 Diesing. 



Polystomidce. — This family comprises a variety of remarkable 

 genera characterized by their possessing in common several suckers 

 which, for the most part, border the posterior extremity of the 

 body ; but, in respect of form and general appearance, they are 

 often widely dissimilar. I accept the family as an equivalent to 

 Dujardin's first group of trematodes which he termed " Onchoboth- 

 riens," rejecting only his genus Diporpa, since we aU know from 

 Yon Siebold's researches that this form is a juvenile condition of 

 Diplozoon. In this family Van Beneden includes the genera 

 Galceostoma and Gyrodactylus, but I propose to place these singular 

 organisms in a separate family. In all the Polystomes we have a 

 more or less ramified intestine, but the reproductive organs conform 



