424 FRESH-WATER BIOLOGY 



251 (252) Anterior part of body hemispherical to cup-shaped; two lateral 



sucking discs also present. . . Cercaria flabelliformis Faust. 



Typical tetracotyle form. Body length 0.48 to 0.56 mm., width 0.44 mm. Animal slipper- 

 shaped, with two posterior and two lateral lappets around sucking discs. Anterior part of 

 excretory system fan-shaped. In rediae or encysted in tissues in liver of Physa gyrina Say, 

 Bitter Root River, Corvallis, Montana. 



Compare 167 (168) in this key. 



252 (251) Anterior part of body lamellate or only slightly patelliform. 



Cercaria ptychocheilus Faust. 



Hemistomum larva. Body length 0.48 to 0.63 mm., width 0.17 to 0.37 mm. Posterior 

 portion abbreviated. Atrial chamber posteriad, well-developed. Varying number of mu- 

 cous glands, situated in posterior part lateral to genital atrium, empty into the latter. En- 

 cysted within semitransparent ovoid membrane, with discoid attachment to mesentery of 

 Ptychocheilus oregonensis Richardson, Bitter Root River, Stevensville and Carlton, Montana. 



The encysted form described by Faust is really a Diplostomulum, i. e., the stage succeeding 

 the true Cercaria. Compare 168 (167) in this key. 



CESTODA 



The cestode or tapeworm is as the name suggests more or less 

 like a band or ribbon, and in the majority of cases the band is 

 subdivided by cross-markings into a series of links or proglottids. 

 In a few primitive tapeworms the body consists of but a single 

 link and the general appearance is so similar to that of the fluke 

 as to make distinction difficult. In some other cases, especially of 

 fish tapeworms, the ribbon-like body is not subdivided by ex- 

 ternal cross-markings, but the internal structure shows the poten- 

 tial presence of proglottids, for the organs are multiplied suc- 

 cessively in the undivided body as they are in the segmented body 

 of the ordinary tapeworm. Most tapeworms are distinctly flat- 

 tened so that one may speak of surfaces and margins. A few 

 species are, however, so nearly circular in cross section that it is 

 difficult to use such terms. Abnormal individuals of the flat- 

 tened species have been described which are three-cornered or 

 prismatic in cross section; these represent partially fused or par- 

 tially split chains. 



One can usually recognize two or three fairly distinct regions in 

 the ordinary tapeworm: the head or scolex, the neck, and the chain 

 or strobila. The head is more or less enlarged, globular or oval, 

 and not infrequently provided with an apical extension designated 

 a rostellum, which in some forms is held withdrawn in a pocket 

 under most circumstances. The head is commonly supplied with 

 suckers and sometimes hooks also by which it attaches itself to the 



