PARASITIC FLATWORMS 411 



167 (168) Larval forms with an oval sucker-like depression on each side of 



the oral sucker Tetracotyle Filippi 1854. 



Body pyriform or oval. On each side of oral sucker an oval groove, not muscular, with 

 pores of special (cystogenous ?) glands. 



Encysted in mollusks and vertebrates. European forms belong to various species of Strigea. 

 Ruttger found these larvae in Limnaea stagnalis and fed them to ducks; ten days later he 

 obtained mature holostomids (species not given). Leidy recorded T. typica from Limnaea 

 catascopium and Physa heterostropha (cf. 251 in this key). Other und escribed species encysted 

 in North American frogs. 



1 68 (167) Larval forms without sucker-like depressions at the side of the 



oral sucker Diplostomulum Brandes 1892. 



Body flattened with lateral margins turned ventrad in anterior region; short tip represents 

 posterior region. On anterior margin near oral sucker group of gland pores on each side. 



Several species encysted in body of fishes, or free in optic bulb of similar hosts. Belong to 

 various Hemistomum species (cf. 252 in this key). 



A form is frequent which has been identified as D. cuticola (v. Nordmann), the larva of 

 Hemistomum denticulatum (Rud.) common in Europe. It has been reported from sunfish, 

 perch, bluegill, pumpkin-seed, minnows, horned dace, rock bass, small-mouthed black bass, 

 pike, and other fish from Canada to Iowa. Not a few of the larger cysts contain two worms, 

 one usually much smaller than the other. 



Cooper found a form in the optic lens in young Micropterus dolomieu which he identified as 

 Diplostomulum wlvens (von Nordmann) . 



169 (i) Larval forms; sexual organs entirely wanting or at most only partly 



developed 170 



A few encysted forms are described that contain eggs and are apparently sexually mature. 



170 (171) Young flukes, encysted or free, always without caudal appendage. 



Agamodistomum Stossich 1892. 



Many immature forms whose relationship to adult types has not yet been determined. 

 The group is artificial, temporary, and collective, including all agamic flukes with two suckers. 

 Agamic forms in other groups have been given special names as noted in connection with the 

 description of the adults. 



Forms of this sort are mentioned frequently without specific names. Named forms are also 

 on record, e.g. A. apodis (Packard 1882) from the ovisac of Apus from Kansas, a unique record 

 of a distome in a phyllopod crustacean, but without data adequate to fix the species. 



All forms described as encysted cercariae belong in this subdivision rather than in the next 

 since the two marks of distinction between the two are the tail, which is cast off when the larva 

 encysts, and the cystogenous glands, pure larval organs, that are emptied in this process and 

 disappear. 



Various species which belong here have been recorded without description under other 

 names as " Heterostomum echinatum Diesing" of Leidy from the oviduct of Paludina "quite 

 common," and Distomum centrappendiculatum of the same author from Helix arborea. 



171 (170) Caudal appendage present, usually simple, sometimes modified, 



even greatly reduced, rarely absent. . Cercaria . . 172 



No hard and fast line can be drawn between this group and the last since a few tailless cer- 

 cariae are known. Furthermore the transition in any case will be instantaneous when the 

 cercaria under stimulation casts off its tail, which happens normally as well as in cultures. 



Small, barely visible, microscopic, free-living forms of simple trematode structure, having 

 a triclad alimentary canal. A tail, single, double, branched, setose, or otherwise modified is 

 nearly always present, and is the efficient organ of locomotion. Rarely the tail is rudimentary 

 or entirely lacking and the form can be classified here only by its strong resemblance in other 

 features to the tailed larvae. The reproductive organs are always rudimentary, and sometimes 

 entirely wanting. At most one can distinguish masses or cords of cells that indicate the loca- 

 tion of future organs. Faust has found that these agree fully with adult conditions. Promi- 

 nent larval organs are the stylet, a boring spine in the anterior tip above the oral sucker, simple 

 eyes appearing as pigment spots on the anterior dorsal region usually near the brain, conspicu- 

 ous dermal glands that in one group are designated stylet glands and in another are assigned a 

 cystogenous function and are perhaps always digestive in character; they present varied features 

 in different species. All of these constitute useful specific characters. 



Very few North American species have been described and the brief records that exist are 

 in most cases adequate only for the definition of groups rather than species in the true sense. 

 Most of the following subdivisions of the key are to be regarded in that light. 



Some names in use like Cercaria bilineata Hald. can have even no general significance since 

 the original reference contains no data that will fix the species. 



