496 Gross Anatomy of Campeloma. 



C. coarctatum Lea and C. exilis Anthony. The same unfortunate 

 cause of synonomy has led, in the genus Unio, to the erection of 

 more than a hundred spurious species, in certain cases the females 

 serving as a basis for not less than five specific names. The form 

 of a shell in so extremely variable a group is certainly a very mis- 

 leading character. 



Digestive Organs. The buccal cavity opens on the middle side 

 of the rather short proboscis near its base. Near the oesophagus 

 (III., Fig. 3) and upon the floor of the buccal mass lies the radula. 

 This is a small, narrow, chitinous organ, beset with numerous trans- 

 verse rows of teeth, arranged according to the formula 3.1.3 (Fig. 

 3, Plate VII., and Fig. 4). The dentition is therefore tsenioglossate. 

 On either side, near the posterior end of the buccal cavity, open 

 the ducts of the salivary glands (II., Fig. 3). These are small 

 racemose paired glands, dirty white in color, and lie close upon the 

 oesophagus. Including their ducts, they are about 1.5 mm. in 

 length, and nearly or quite .25 mm. in width. The oesophagus is 

 long, irregularly winding, placed upon the floor of the branchial 

 cavity, and opens, into a somewhat capacious stomach (IV., Fig. 3), 

 near the middle of the whorl next the body-whorl. The intestine 

 is of nearly the same size as the oesophagus, and does not enlarge 

 until the opening of the biliary duct is passed (V. and VII., 

 Fig. 3). At this point it is coiled upon itself to the left, forming 

 what may be called the right duodenal fold, immediately under 

 which lies the testis, as stated above. Turning again to the right, 

 it is there directed forward, becomes slightly enlarged, forming the 

 rectal portion of the intestinal canal (VIII., Fig. 3), which opens 

 into the branchial cavity near the margin of the mantle on the right 

 side (IX., Fig. 3). The liver (VI., Fig. 3) is a very large gland- 

 ular body, completely filling the first two and a half to three whorls 

 of the shell. Its contents are discharged into the duodenal portion 

 of the intestine near the position of the heart. In color it is orange- 

 red, and is somewhat larger and darker in the male than in the 

 female form. This organ, like all other portions of the animal 

 which lie next the shell, is in versed by a thin membrane, contain- 

 ing pigmentary matter, the membrane itself being a continuation 

 of the mantle. 



Respiratory Apparatus. The branchial cavity is large, extend- 

 ing backwards throughout nearly the whole length of the body- 

 whorl. It opens towards the right side, its left margin being just 



