174 Introduction to Animal Morphology. 



from the oviduct to the testis to ensure self-impregna- 

 tion. The sex-organs always open medio-ventrally. 

 Gyrodactylus elegans is viviparous. The eggs are 

 often of odd shapes, and may be developed directly or 

 metagenetically. The embryos of some are non- 

 ciliated (D. tereticolle, variegatum). 



There are two sub-orders : — 



I. Monogenasa — eggs large, often angled, pillared, or 

 filamented, with thick shell ; development direct ; young not 

 ciliated. Ecto-parasites on the skin and gills of fishes, 

 crustaceans, &c. This includes six families : — i. Octocoty- 

 lidae — long ; hinder part of the body lengthened as a tongue- 

 like lobe, armed with eight suckers in two rows ; two suckers be- 

 side the mouth ; body armed with hooks about the genital pore. 

 Microcotyle has small suckers and posterior hooks ; Gastro- 

 cotyle has over thirty little suckers posteriorly, and the front 

 of the body is narrow ; Phyllocotyle has two anterior and six 

 hinder suckers, and a tail armed with a sucker and hooks. 

 Anthocotyle is club-shaped in front, with two small suckers, 

 and has posteriorly six small, stalked suckers ; at the hinder 

 point of trisection are two oval side suckers, with frilled 

 hinder margins, and each ending externally in a stalked 

 sucker. In Diplozoon, a carp parasite, the individuals, when 

 they attain maturity, unite in pairs (like the Siamese twins) 

 at their hinder point of trisection, becoming x-shaped. Each 

 single individual has two fore suckers, and two hinder areas 

 each with four suckers. The sex-openings are behind the 

 point of union. The cirrus is spiral, 2 J times as long as the 

 whole body. Choricotyle has posteriorly eight suckers on 

 long stalks, a crown of hooks round the sexual opening, and 

 two small front suckers. 2. Gyrodactylidae — minute, fish 

 parasites, with two anterior suckers ; the hinder part of the 

 body dilated, margined by small hooks, and armed with two, 

 long, recurved hooks ; pharynx protrusible, with a papillary 

 margin anteriorly; water-vascular stems 4, of which only two 

 end in the pore. Gyrodactylus is found in carp, &c. ; in 

 these, second embryos may be developed within the first, 



