64 PLATYHELMINTHES TREMATODA chap. 



hut in all probability after entering a host (Fish, Amphibian, 

 Mollusc), it undergoes a gradual change into what has long been 

 known as a Tetracotyle, from the frequent presence of four 

 ms (sometimes only three) adhering organs. 



Fig. 31 exhibits a species which is abun- 

 dant in the lens and vitreous humour of 

 the eye of the Perch. Its further history 

 is not known, but presumably the Perch 

 is presently devoured by the final host in 

 which the Diplostomum attains maturity. 

 Thus the Holostomatidae are " metastatic " 

 (Leuckart), their (probably) direct develop- 

 ment requiring the presence of two hosts. 1 

 The other Digenea, the life-histories of 

 which are known, belong to the Disto- 

 FiG.Sl.-m P lostomum(Tdra- matidae and Amphistomatidae, and we may 

 cotyle) voivens. (After distinguish the steps by which the com- 

 lv, ContSiie excretory plex life-history of the liver-fluke (Dis- 

 vesicie ; d, intestine ; e, tomum hepaticum) has been brought about, 



calcareous bodies in ex- . " , 



cretory tubules ; ex.<>, by a consideration ol that of Uistomum 



excretory aperture ; gl, maCTOStommn. 

 glandular adhesive body ; 



ms, oral sucker ; ph, Bistomum macrostomum. This form 

 pharynx ; vs, ventral occurs [ n ne intestine of several common 



sucker. 



Passerine birds. It is remarkable not only 

 for the large oral sucker, but also on account of the position of 

 the common genital pore at the hinder, and not as usual, at the 

 anterior, end of the body (Fig. 32, A). The eggs pass out 

 through this pore, and are discharged with the bird's excrement. 

 Should a certain snail (Succinea putris) happen to rasp off the 

 epidermis of a leaf upon which the faeces have fallen, the eggs 

 are swallowed and a minute active larva is set free (Fig. 32, B). 

 This penetrates through the thin wall of the digestive tract of 

 the snail, and passing into the connective tissue, throws off its 

 cilia and assumes the shape of Fig. 32, C. This sporocyst, as the 

 larva is now termed, grows rapidly in all directions (Fig. 32, D) at 

 the expense of the snail's tissues, until it becomes impossible to 

 separate parasite and host completely. Those branches which 



1 Braun, Bronn's Thierreichs, Bd. iv. p. 792 ; Leuckart, Parasiten d. Menschen, 

 11 Abth. p. 158 ; Brandes, in Spengels Zool. Jahrb. Syst. Abtheil. Bd. v. 1890, 

 p." 849 ; v. Nordmann, Mikr. Beitr. i. Berlin, 1832. 





