HETEROCOTYLEA. 



233 



Order 1. HETEROCOTYLEA = MONOGENEA. 



Body variously shaped. Anterior end with or without suckers; 

 hind end always with a suctorial organ. The ectodermal epithelium 

 is transformed into a cuticular layer except in the Temn6cepJialidae 

 and in the lateral suckers of the Tristomidae. Eyes are present. 

 The openings of the excretory organs are usually paired, dorsal and 

 anterior ; rarely single and posterior. Always hermaphrodite ; male 

 and female openings separate or united, 

 ventral and usually anterior. A paired or 

 unpaired vagina is generally present, opening 

 ventrally or laterally, or more rarely dorsally. 

 Eeproduction sexual; development direct, 

 without metagenesis or heterogamy. For the 

 most part external parasites, on the integu- 

 ment, or in the mouth, nasal passages, 

 branchial cavity, in some cases in the urinary 

 bladder, of Fishes, Amphibia, Reptiles, and 

 Crustacea. They are usually hatched in the 

 locality inhabited by the mother. Some- 

 times the development is a metamorphosis, 

 and the young live in another place. 



The development of Polystomum integcrrimum, 

 parasitic in the bladder of the frog, is the Lest 

 known, owing to the researches of E. Zeller* (Fig. 

 186). The production of eggs begins in the spring, 

 when the frog awakens from its winter sleep and 

 proceeds to pair. It lasts from three to four weeks. 

 It is easy then to observe the Polystoma in the 

 process of reciprocal copulation. When the eggs 

 are being laid, the parasite forces the anterior end 

 of the body with the genital opening through the 

 mouth of the bladder nearly as far as the anus. 

 The development of the embryo takes place in 

 water, and occupies a period of some weeks, so 

 that the young larvae are not hatched until the 

 tadpoles have acquired internal gills. The larvae 

 (Fig. 186) are Gyrodactylus-like, and possess four 

 eyes, a pharynx and alimentary canal, as well as 

 a posterior disc for attachment, which is surrounded by sixteen hooks. They 

 possess five transverse rows of cilia, three are ventral and anterior, two dorsal 

 and posterior. There is also a ciliated cell upon the anterior extremity. The 

 larvae now migrate into the branchial cavity of the tadpole, lose their cilia, 

 and are transformed into young Polystoma by the formation of the two median 



* Z.f. w. Z., 22, 1872. 



FIG. 185. Polystomum inte- 

 gerrimum (after E. Zeller). 

 mouth ; Go genital open- 

 ing ; D intestine ; W lateral 

 papillae bearing openings 

 of vaginae (Laurer's canal) ; 

 Dg yolk - gland duct ; S 

 sucker; Ov o vary ;H hooks. 



