66 



MUSSEL FAUNA OF MAUMEE EIVER. 



of the ventral surface, four-sevenths of the entire length from the 

 anterior end, circular, with radiating ridges and four-sevenths the 

 diameter of the body. Posterior end of body with traces of the zig- 

 zag longitudinal ridges seen on the tail of the young. Mouth in the 

 center of the anterior sucker, opening into a spherical or transversely 

 elliptical pharynx close behind the sucker and one-third its diameter. 

 From the pharynx two digestive tubes lead back to the posterior end 

 of the body, one on each side, each a little narrower than the phar- 

 ynx, of nearly uniform diameter throughout, and curved outward 

 parallel to the lateral body margin. They reach nearly to the poste- 

 rior end of the body, where each ends in a network of intercellular 

 spaces and tubes, the excretory organ. 



The newly escaped cercarium is about 1.05 millimeters in total 

 length and 0.2 millimeter wide. The length and width are con- 

 stantly changing, according as the organism is stretched out or 

 contracted. 



The tail is no longer an organ of locomotion ; it is very large and 

 unwieldy and easily detached ; most of the cercaria shed it soon after 

 emergence from the sporocyst, so that it is evidently a vestigial organ. 



After shedding the tail the organism, which now has the typical 

 form of a Distomum^ moves up to the umbonal region of the mussel 

 and acquires a pinkish color. From the umbones it spreads in di- 

 verging lines postero-ventrally and antero-ventrally and finally fills 

 in the space between. Where it is located it profoundly affects the 

 nacre, staining it a salmon color, and frequently giving it a diseased 

 appearance. The presence of this distomid in Anodonta and its re- 

 lation to the salmon-colored nacre was first pointed out by H. L. 

 Osborn« and further discussed by H. M. Kelly.^ 



In its general appearance and in its life history this organism 

 agrees very closely with Distomum diiplicatum as described by von 

 Baer, who gives a detailed description of the sporocyst and a history 

 of the escape of the cercaria and its subsequent behavior.^ It differ^ 

 from that species in the fact that the tail is not clavate, is longer 

 than the body, straight, and not pediculate. It differs from the 

 cercarian genus Rhophalocerca Diesing, formed to include D. dupli- 

 oatum^ only in the fact that the tail is not clavate. Von Baer de- 

 scribes what appeared to him a form of reproduction in this distomid, 

 but from what is now known of related forms it appears probable 

 that he misinterpreted appearances. Distomum. duplicatum (known 

 also as Rhophalocerca tardigrada Diesing) is of special interest as 

 the organism first shown by De Filippi to be the cause of pearls in 



"Zoological Bulletin, vol. i, no. 6, 1898. 



^ Bulletin Illinois State Laboratory Natural Flistory, vol. v^ art. viii. 

 c Nova Acta Pliysico-Medica Academia> Csesarese Leopoldino-Carolinae, Natural Curio- 

 sorum, vol. xiii, pt. 2, 1827. 



