50 THE TREMATODA 



ORDER 1. Heterocotylea, Monticelli ( = Polystomea, Leuck. = Pecto- 

 bothrii, Burm. = Monogenea, v. Ben.). 



Trematoda, in which there is a large posterior, ventral, terminal 

 adhesive organ in addition to a pair of anterior suckers in relation to the 

 mouth ; the latter may be absent. The posterior apparatus consists 

 either of a single sucker, usually of large size, which is generally divided 

 by radial ridges into a number of compartments ; or these ridges may be 

 so extensively developed as to give rise to a number of separate suckers 

 set upon a caudal disc or " cotylophore." This posterior apparatus is 

 very usually provided with chitinous booklets. 



Eye-spots are not unfrequently present. The excretory system com- 

 municates with the exterior by a pair of pores laterally placed on the 

 dorsal surface, near the anterior end. The male and female ducts nearly 

 always open by a common pore. A third genital duct, known as the 

 vagina, is usually present, with an external aperture independent of the 

 uterine pore. The members of the group are nearly all ectoparasitic, 

 and development takes place without the intervention of an intermediate 

 host, and without any intercalated asexual reproduction (hence "mono- 

 genetic "), so that from each egg laid only one new fluke is produced. 



For accounts of the anatomy of various genera, see van Beneden, 5 ; 

 Goto, 19 ; Cerfontaine, 14. 



FAMILY 1. MONOCOTYLIDAE. Posterior sucker usually small ; anterior 

 suckers absent ; the common genital pore median. Pseudocotyle^ v. Ben. 

 and Hesse, on skin of Selachians. Calicotyle, Dies. ; G. kroyeri, Dies., 

 in cloaca of male Raia. Monocotyle, Tasch. (Fig. III. 2) ; M. mylio- 

 batiSj Tasch., on Myliobatis aquila. FAMILY 2. THISTOMIDAE, with one 

 large posterior sucker, with or without compartments ; with or without 

 booklets ; a pair of anterior " lateral " suckers ; male and female ducts 

 usually open by a common pore situated anteriorly, usually on the left 

 side ; vagina and its aperture single on the left side. Parasitic on gills 

 and skin of marine fish. Nitzschia, v. Baer ; N. elongata, Nitzsch, gill 

 cavity of Sturgeon. Epibdella, Blv. ; Phyllonella, v. Ben. and H. ; P. 

 soleae, v. Ben. and H. ; Tristomum, Guv. (Fig. II. 1) ; Acantliocotyle, 

 Montic. ; A. lobianchii, Montic., on skin of Raia clavata. Encotyllabe, 

 Dies. ; E. nordmanni, Dies., on nostril of Bream. Udonella, Johnston 

 (Fig. II. 5), on parasitic Crustacea. Ediinella, v. Ben. and H. ; 

 Ptqronella, v. Ben. and H. FAMILY 3. POLYSTOMIDAE. The posterior 

 adhesive organ is formed by six or eight suckers on a caudal disc or 

 " cotylophore " which is usually armed ; two anterior " buccal " suckers 

 communicating with the oral cavity are usually present ; vagina single 

 or paired; common genital pore median. On gills of fishes, skin and 

 bladder of Amphibia and Reptiles (see Goto, 19). SUB-FAMILY 1. 

 OCTOCOTYLINAE, with four to eight posterior suckers ; a pair of " buccal " 

 suckers ; genital booklets. A. Suckers, four on each side. Octobothrium, 

 Lkt ; 0. alosae, v. Ben. and H., on gill of Shad. Diplozoon, v. Nordmann. 

 The genital ducts are so arranged that the male duct of each animal 

 becomes continuous with the vagina of the other (Zeller, 50), or with 

 vitelline duct (Goto, 1 8). D. paradoxum, v. Nordm., on the Minnow ; 



