PARASITIC FLATWORMS 379 



28 (2) Organs of attachment one or two suckers of which the anterior is 



always single and median; without chitinous hooks or 

 anchors; accessory suckers rare. Subclass Digenea . 29 



Excretory organs empty by a single pore at or near posterior end. Uterus usually long, 

 containing masses of eggs, rarely only a few. Development complex, with alternation of 

 hosts and most often also of generations. 



With rare exceptions adults endoparasiticin visceral organs, usually alimentary system of ver- 

 tebrates. Isolated adults occur in mollusks and insects which are the normal hosts for young 

 stages. 



For key to free living larval stages see section on Cercaria, 171 (170) in this key. 



29 (30) Anterior sucker not perforate; mouth on mid-ventral surface; no 



oral or ventral suckers. . Order Gasterostomata Odhner. 



Pharynx and esophagus present. Intestine sacculate, simple. Vitellaria lateral in anterior 

 region of body. Germ glands behind intestine, in posterior region. Testes two; cirrus elon- 

 gate; pore ventral near posterior end. Ovary simple, opposite or in front of anterior testis. 



Single family Bucephaledae Poche 1907. 



Only genus known Bucephalus von Baer 1826. 



Anterior end bears large sucker with ventral orifice and small muscular 

 papillae at lateral angles. 



The adult, better known as Gasierostomum, has been reported only from 

 Canada though to judge from the abundance of the characteristic two-tailed 

 cercaria it must occur frequently in other regions. 



Stomach, intestine, and ceca of black bass and BoUosoma nigrum. Early 

 stages encysted in young black bass, rock bass, perch, and minnow. * 



Cercariae parasitic in Unionidae, especially in sex organs. Pennsylvania, 

 Illinois, Iowa, Canada. Not common; occurring in fifteen species of Unionidae 

 out of forty-four examined; in susceptible hosts only 4 per cent of individuals 

 affected (Kelly). 



Representative North American species. 



Bucephalus pusillus (Stafford 1904). 



Fig. 660. Bucephalus pusillus. Ventral view. X 75. (After Cooper.) 



30 (29) Mouth at or near anterior tip of body ordinarily surrounded by oral 



sucker; another sucker if present median, behind mouth 

 on ventral surface or at posterior end. 



Order Prosostomata Odhner . . 31 



31 (36) Intestine simple, rhabdocoel; oral sucker very poorly developed; 

 ventral sucking organ a powerful, conspicuous, adhesive 

 disc or a series of smaller suckers. 



Suborder Aspidocotylea MonticeUi. 



Terminal or subterminal mouth surrounded by funnel-shaped expansion of skin, but not by 

 true sucker. Holdfast organs ventral, usually in form of large sucking disc distinctly set 

 off from body and subdivided into numerous sucking alveoli, but never carrying chitinous 

 hooks or anchors; or in place of disc single series of small disconnected suckers. Alimentary 

 canal simple, rhabdocoel. Sexual organs simple. Development with or without alternation 

 of hosts and generations. Endoparasitic, or rarely ectoparasitic, in mollusks and cold-blooded 

 vertebrates. 



Forms not numerous, little known, grouped together at present into 

 a single family. . . Aspidogastridae Poche 1907 . . 32 



