PARASITIC FLATWORMS 



381 



35 (34) Mouth terminal, surrounded by expanded buccal disc. 



Cotylogaster Monticelli 1892. 



Ventral disc composed of single median row of 

 grooves greatly elongated transversely and sur- 

 rounded by marginal row of small, circular alveoli. 

 Mouth in center of discoidal expansion of anterior 

 tip of forebody. Long prepharynx and esophagus. 

 Ovary and two testes just behind it form linear 

 series posterior to center of body. Laurer's canal 

 present. Embryo with large posterior sucker; 

 development unknown. 



Parasitic in intestine of fishes. 



Single North American species. 

 Cotylogaster occidentalis Nickerson 1900. 



In intestine of sheepshead (Aplodinotus grun- 

 niens), Minnesota. Rare. 



FIG. 663. Cotylogaster occidentalis. A. Lateral 

 view of an entire alcoholic specimen in which the an- 

 terior portion is retracted. X 8. B. Diagram show- 

 ing relation of organs as seen from the dorsal .side, the 

 animal being represented as straightened horizontally 

 with the dorsal cone projected backward. Magnified. 

 (After Nickerson.) 



36 (31) Intestine forked; oral sucker distinctly developed; ventral sucker if 

 present simple 37 



In one genus (Cryptogonimus) the ventral sucker consists of two small acetabula close 

 together; in a few genera it is more or less intimately connected with a genital sucker sur- 

 rounding the sexual pore, but in no case does it consist of a series of small sucking organs or 

 have a complex, many-parted structure. 



The forms embraced under this heading in the four suborders which follow, stand in sharp 

 contrast with those of the suborder Aspidocotylea just preceding. In fact the latter are so dis- 

 tinct in general appearance, in structure, and in development, that they have regularly been 

 grouped heretofore apart from the orders which follow. They were generally included under 

 the Monogenea (p. 374) until Monticelli revived the original view that they should be regarded 

 as an independent subdivision of equal rank intermediate between the Monogenea and the 

 Digenea (p. 379). Their very recent inclusion in the latter group has been well justified; yet 

 even with that the striking differences noted above must be kept clearly in mind. 



The forms which follow fall naturally into four groups ranked here as suborders; they are 

 easily distinguished by a single external feature, the adhesive apparatus, consisting of suckers 

 which in number and arrangement are characteristic of each group. Thus the holostomes have 

 in addition to the oral and ventral suckers a special adhesive organ behind the latter. This 

 special organ is variable in form and character. In the amphistomes one finds an oral and a 

 terminal sucker, but no other adhesive organs. The distomes possess an oral and a ventral 

 sucker but none further back, while finally the monostomes have only one sucker and that is 

 circumoral in location. 



These long recognized groups are already beginning to break up under the influence of more 

 careful study, and as noted in the next section steps have been taken to eliminate the monostomes 

 as an independent subdivision, distributing its members among other groups. 



