10 The Molluscau Family Planorhidae 



to contain small particles of siliceous matter, evidently sand taken in with 

 the food to serve for the breaking up of the ingested food. 



On the under side of the pyloris, near the duct of the liver, there is a 

 finger-like process or blind sac which appears to be the stylotheca con- 

 taining the crystalline style. This blind sac is present in all species of the 

 family examined. Internally the sac contained a gelatinous substance or 

 was empty. The liver or digestive gland covers the posterior part of the 

 stomach region in its natural position. It has two unequal lobes which 

 form the posterior part of the animal and in which the ovotestis is con- 

 cealed. The liver is composed of a great number of small lobules, more or 

 less digitiform. It is well supplied with blood vessels which form a plexus 

 on and in the organ. A large duct enters the pyloric portion of the stomach 

 at the point of junction with the blind sac. 



The intestine is very long. It bends around the stomach from the end 

 of the pyloris and passes backward for a distance. It then forms a single 

 loop wdiich bends forward and extends to the fore part of the animal on 

 the left side (the right side in the Lymnaeidae). The rectum, the greatly 

 enlarged portion of the intestine, supports a central ridge or crest on the 

 upi5cr side which extends to and over the pseudobranch. The anal opening 

 is always near this ridge on the pseudobranch, to the right and above this 

 organ. 



Variation in the digestive system is noted only in the form of the 

 stomach region, short and rotund in some species (plate 48, fig. 1, Helisoma 

 trivolvis) or elongated and narrow in other species (plate 48, fig. 8, 

 Bathyomphalus contortiis) . The loop of the intestine sharply borders 

 the stomach in some species (fig. 1) though in some species it extends 

 forward without a loop (fig. 8). 



[1] The Jaw 



The mouth of the Planorbidae is provided with two important organs 

 which afford characteristics for classification, the jaw and the radula. The 

 horny jaw lies in the upper part of the mouth at the fore end of the buccal 

 sac (plate 70, fig. 13). As in Lymnaea, the mouth is somewhat T-shaped 

 in repose. In the upper part of the T lies the superior jaw, which is wide 

 and low, of a brownish color, striated vertically on its front part, the 

 striations producing a saw-like condition on its lower, cutting edge (plate 

 49, fig. 1, Helisoma anceps) . The lateral jaws, two narrow, brown, horny 

 ribbons, emargin the lips or sides of the mouth. They are connected with 

 the lateral margins of the superior jaw by a narrow projection from the 

 upper part of the lateral jaws. These side jaws form a question-mark 

 figure, as will be noted in the figures on plate 70. The jaws, superior and 

 lateral, are attached to a cartilage which in turn is attached to the roof of 

 the mouth. The jaws are moved by several distinct muscles. 



The jaws of the Planorbidae fall into two principal types or variations. 

 The most common is the group described above, with large superior jaw 

 and two smaller lateral jaws. Another type is horseshoe-shaped and in this 

 there is but one division which appears to represent the fusion of the 

 lateral and superior jaws of the first type. The striations of the superior 

 jaw of the first group become separate vertical plates in the second group, 

 producing a segmented jaw. This segmentation extends to the lower part 



