General Morphologij 9 



which have a horny stylet or fleshy papilla at the end of the verge 

 {Gyraulus, Anisus, and other genera). 



Another feature observed in connection with the penial gland is that 

 the i)enial complex is often seen in a bilobed condition, the gland being 

 pushed upward in the preputium causing the vergic sac to appear as 

 though attached to the side of the preputium. This condition occurs mostly 

 in HcUsoma (plate 25, figs. 1, 6, 7, 9; plate 24, figs. 1, 2). The evolution 

 of this feature was observed in specimens of Helisoma ccunpanulatum and 

 is shown on phite 27, fig. 7 being the nonual form and figs. 6, 5, and 8 

 showing the gradual change to the bilobed condition. 



Much remains to be learned about the function of the penial gland. 

 Observations of the living snail in aquaria should help to solve some of 

 the questions. It may be observed, however, that this is not so easily ac- 

 complished as might be supposed. More than a dozen acjuaria were kept 

 under observation by the writer for several years, and while the animals 

 were seen in apparent coitus on several occasions, the protrusion of the 

 gland was not seen. Perhaps histological studies would help, especially 

 before and after breeding. The subject is worthy of a doctor's thesis. 



As was recorded in the study of the family Lymnaeidae (Baker, 1911), 

 there is great variation in the form of the genitalia of the genera of the 

 family Planorbidae. As in the Lymnaeidae, also, this modification occurs 

 mostly in the male organs, principally the penial complex, but includes 

 also the ]:)rostate and seminal vesicle. The female organs are uniform in 

 position and shape, only the spermatheca and its duct vary in form 

 and length to any degree. The male organs, therefore, afford useful features 

 upon which to base a natural classification of the family, since the 

 characters of the different groups are constant and uniform. 



[b] The Digestive System 



The digestive system of the Planorbidae is similar in many respects to that 

 of the family Lymnaeidae. The system naturally divides into three parts, 

 the first consisting of the buccal sac, the oesophagus, and the salivary 

 glands, the second part the region of the stomach with the liver connec- 

 tions, and the third part the intestine and rectum (see fig. 1, plate 48). 



The buccal sac is usually pear-shaped, the oesophagus entering it at 

 the upper middle portion. There are retractor and protractor muscles and 

 nerves received from the buccal ganglia. The salivary glands are paired, a 

 duct entering the buccal sac on each side near the oesophageal connection. 

 The salivary glands, usually cylindrical and elongated, are composed of 

 many small lobules. The two glands are joined behind and form a con- 

 tinuous loop or ring. These organs are very uniform in the different species 

 of the family (see plate 70, figs. 12, 13). The oesophagus is usually rather 

 long (about one-fourth as long as the intestine), beginning as a tube of 

 small diameter and enlarging as it approaches the stomach region. 



The stomach is a large muscular body composed of three parts, first 

 the crop, w'hich is an enlarged portion of the oesophagus, then the gizzard, 

 which is an indistinctly bilobed body of large diameter, and third the 

 pyloris, or opening into the intestine. The gizzard is not as distinctly 

 bilobed as in Lymnaea but is of the same nature. When ojiencd it is found 



