144 The Molluscan Family Planorhidae 



end of the organ. A cross section of the kidney made at about the middle 

 portion (plate 44, fig. 9) shows the ridge to be rather short and narrow 

 and placed at about the middle of the kidney. The lumen is rounded and 

 the veins at each end are large and placed at the center of the margin of 

 the lumen. In the race lentum (plate 44, fig. 11), a section near the lower 

 end shows a large, ovate lumen with the veins at the lower corners. The 

 ridge is approximately like that feature in trivolvis. In macrostomum, the 

 largest race of trivolvis, a section made near the lower end shows a small, 

 much-elongated lumen with the veins at the lower lateral corners (fig. 10). 

 The whole kidney is much larger and much thicker with a relatively small 

 ridge. 



The species and races of Pierosoma show considerable variation in the 

 form of the lumen, and especially in the form and position of the ridge, 

 which is found in all species of Helisoma and its subgenera. Several of 

 these are figured on plate 44 and need no special comment. Those figured 

 are: pilsbryi, section near middle, fig. 12; infracarinatum, near middle, 

 fig. 17; magnificum, near middle, fig. 14; corpulentum, near middle, fig. 

 15; multicostatum, section near lower end, fig. 16; truncatum, near middle, 

 fig. 13; subcrenatimi, near middle, fig. 21; plexatuni, section near lower 

 end, fig. 22; (compare figs. 21 and 22 and note difference in ridges); 

 horni, near middle, fig. 23; occidentale, near middle, fig. 18; depressum, 

 near middle, fig. 19 (note difference in two races); traskii, near middle, 

 fig. 20; calif orniense, near middle, fig. 24; oregonense, near lower end, fig. 6. 



The kidney of corp^dentum (fig. 3) is somewhat shortened and bent 

 toward the pericardium. The ridge is broad at the upper part and then 

 narrowed for the greater part of its length. A cross section is shown in 

 fig. 15. Compare this with the kidney of trivolvis, figs. 2 and 9. 



The kidney of calif orniense is figured on plate 45 (fig. 4). It is rela- 

 tively short and variable in width. A cross section made through the upper 

 part (fig. 5, at B in fig. 4) shows the roundly-ovate lumen occupying the 

 greater part of the kidney, with the veins at the middle on each side. A 

 section made near the lower end of the kidney (fig. 6, at A in fig. 4) shows 

 a very wide and flattened lumen with the veins at each end, and a high, 

 pointed ridge. Compare with fig. 21 on plate 44. 



Digestive System. The digestive system of Helisoma trivoh'is (plate 

 48. figs. 1,2) is cliaracteristic of that of all species of Pierosoma. The size of 

 the organs differs in the various species. The oesophagus (OE) is a narrow 

 tube as it leaves the buccal sac, increasing several times in diameter as it 

 passes to the crop portion of the stomach. It is about 10 mm. long. The 

 stomach is composed of a short crop (CR) , a bilobed gizzard (GZ) , and an 

 elongated pyloris (PY). The intestine (IN) makes a complete turn over 

 the oesophagus, extends backward around the liver, performs another 

 loop, and runs forward to the columella muscle. The intestine is 33 mm. 

 long from the end of the pyloris to the rectum. The rectum is 10 mm. long, 

 is larger in diameter than the intestine, and is particularly characterized 

 by the presence of a frilled crest or ridge (RD) which extends the whole 

 length of the rectum and ends over the pseudobranch. In fig. 2, the stomach 

 is seen from below showing the position and form of the blind sac or 

 caecum (BS) which is attached to the pyloris just below the opening of the 

 liA^er (OLV). This blind sac is present in all species of Pierosoma. 



