General Morphology 15 



[g] The Muscular System 



The muscular system of the family Planorbidae does not differ from that 

 of the family Lymnaeidae except in i)ositions due to the sinistrality of the 

 animal. The most important muscles for classification purposes are those 

 of the male generative system. 



C. THE SHELL 



The shells of the Planorbidae are mostly orb-shaped, wheel-shaped, or 

 disc-shaped, rarely becoming a conical spiral as in Lymnaea and Physa. 

 Several physoid types occur, as the Helisoma scalare of Florida. Such types 

 sometimes occur in other species of the family as abnormal or pathologic 

 forms, and have been noted in the genera Helisoma and Carinifex. Scalari- 

 form individuals may also occur in any species and have been noted par- 

 ticularly in the smaller species of the genus GyrauliLS. 



The shell may be closely coiled, like a watch spring, or loosely coiled, 

 and of few or many whorls. It may be so fiat that one wonders how an 

 animal can find enough room to function in so small a space, as Drepano- 

 trema cultratum, or it may be almost globose, as Helisoma pilsbryi. It may 

 be as thin as paper or thick and very solid. 



The shells of the larger speLcies^_especially in_ Helisoma, are carried 

 almost perpendicularly as in jHelisojnq trivolvisjnaa^o^tomum (shown on 

 plate 70, fig. 7) , or it may lie'ahiiost flat as in Planorbida^nksi (shown 

 on plate 70, fig. 11). In all species of the family Planorbidae the ^nital 

 organs are on the left side^ -and the animal is sinistral. But the shells are 

 not all strikingly Sinistral. Such large groups as Helisoma, Planorbarius, 

 and Indoplanorbis are obviously sinistral, usually exhibiting a distinct 

 right and left side- wh^ii ia loconiption. But the smaller species are tipped 

 to one^ side, to the left, and are apparently dextral and are usually de- 

 scribed as pseudodextral or ultradextral. In such species the upper side 

 (right) is umbilical and the lower side (left) is apical. These shells, while 

 truly sinistral, appear as dextral. In most works on this family the shells 

 have all been considered dextral and figured as such. Quite a literature 

 has developed in the controversy as to whether the shells of Planorbis are 

 dextral or sinistral (see Baker, 1896. Jour. Cincinnati Soc. Nat. Hist., 

 19, p. 45). 



In size the planorbids range from the tiny Armiger crista, less than 

 2 mm. in diameter, to the large Helisoma trivolvis macrostomum, which 

 attains a diameter of 30 m m. The whorls may be rounded on the periphery 

 or this region may be sharply carinated or squarely flattened. The lower 

 (umbilical) surface may be so flattened that no umbilical opening is indi- 

 cated, or it may be deeply indented forming a distinct umbilicus. Sutures 

 may be deeply indented or scarcely visible. The aperture is most frequently 

 rounded, ovate, or half-moon shaped. It may be armed with lamellae or 

 entirely free from such obstructions. The outer lip may be thin and sharp 

 or it may be strengthened by a heavy ridge or callus. There may be, also, 

 more or less of a callus on the parietal wall. As most species are planorboid 

 in form there arc few modifications of the columella, as are found in the 

 familv Lvmnaeidae. 



