12 The Molluscan Family Planorhidae 



families and also their common descent from the tectibranchiate stock of 

 marine mollusks. The lateral teeth of the Planorhidae show a wider degree 

 of variation than do the lateral teeth of the Lymnaeidae, indicating, per- 

 haps, that the Planorhidae may be somewhat younger in development. 



In studying the teeth of the Planorhidae it must be remembered that 

 only the unworn teeth show the true characteristics of the radula. Also the 

 teeth must be in their natural position on the radula ribbon and not tilted 

 or otherwise disturbed. 



The Planorhidae are divisible into two groups by the form of the 

 marginal teeth. One group, including the genus Helisoma and other 

 divisions of the subfamily Helisomatinae, have long and narrow reflec- 

 tions reaching far below the base of attachment and with the distal end 

 and the outer margin multiserrate (see plate 57). The other group, includ- 

 ing the genus Planorhis and other divisions of the subfamily Planorbinae 

 as well as some of the subfamily Segmentininae, have short, wide reflec- 

 tions high up on the base of attachment and only a few^ serrations or cusps 

 on the lower margins (see plate 69). 



The lateral teeth are very uniform in both groups, these being mostly 

 tricuspid, the cusps rounded or sharp and dagger-like. In several groups 

 (as Drepanotrema and Segmentina) the laterals are modified by the 

 addition of several cusps, causing the teeth to be 6 or 7 cuspid. These 

 exceptions appear to be due to a splitting of the three cusps of the normal 

 lateral teeth (entocone, mesocone, ectocone) found in the majority of 

 planorbid genera. As with the other organs, the radulae offer sound criteria 

 for classification purposes. 



[c] The Renal System 



The kidney or renal organ (nephridiumt of the family Planorhidae is a 

 large and conspicuous part of the snail's anatomy, occupying a consider- 

 able portion of the body whorl of the animal. It lies to the left of the 

 lung cavity, the genitalia being between the kidney and the ureter (see 

 plate 14, fig. 4, U, NG). Viewed from the ventral side, the kidney consists 

 of two parts, a small upper saccular portion with the pericardium at the 

 right (SK) and an elongated lower portion (TK). A section through the 

 middle of this portion of the kidney (plate 44, fig. 9) shows a central tube 

 or lumen flanked on each side by a large vein, the pulmonary vein on the 

 right side (AP) and the renal vein on the left side (RA). The central 

 tube, the tubular portion of the kidney, is regularly marked by internal 

 septa which project into the lumen for a greater or less distance. The 

 tubular portion is surrounded by thick walls of spongy tissue, the most 

 conspicuous part being a distinct ridge (RK) . To the left of this ridge (in 

 the section) there is another conspicuous ridge on the roof of the mantle. 



At the lower end of the tubular portion is the short ureter (UR). This 

 is a small tube which turns to the left, usually more or less upward, and 

 discharges into the mantle cavity. The figures on plates 44 to 47 show the 

 various forms of ureter and the relationships of the different parts of the 

 kidney. In 1940, Mattox, working on another group of snails, pointed out 

 the fact that the kidney may reflect specific characters. 



A series of sections through the kidney of Australorbis glabratus (plate 

 45, figs. 7 to 11) show the internal form of the kidney at different points 

 in its length. In fig. 1, plate 45, an arrow indicates the place where each 



