LECTURES ON MOLLUSCA. 229 



It steps about, more quickly than most rnollusks, in rocky crannies on 

 the sea-shore. 



Family Otinidje. {Ear Snails.) 



The little shell of Otina could hardly he distinguished from Velutina 

 but the animal closely resembles Auricula. The tentacles are very 

 small ; the foot grooved for looping ; and the mouth cleft vertically. 

 The little creatures live in the same situations as Pcdipes. 



Family ~Lw$2EH>m. (Freshwater Snails.) 



In company with Melanias, Paludinas, and other gill-breathing 

 freshwater Periwinkles, are found in every part of the globe shell-fish 

 which never leave the water, and yet are as truly air-breathers as the 

 whales. They must needs come to the surface occasionally to breathe, 

 where they may be seen gliding upside down, and sometimes letting 

 themselves drop at the end of a glutinous thread. They have short, 

 stumpy tentacles, with eyes on the inner basis, and very broad feet. 

 They abound most in temperate regions. The breathing hole is at 

 the right side of the neck : the vent at the left. They lay their eggs 

 in gelatinous masses on the leaves of water plants which -they devour. 

 The Limncea stagnalis has 110 rows of 111 teeth each, and is said to 

 prefer feeding on decaying animal matter. The shells of Limnaia are 

 thin, with a pointed spire, and a fold on the pillar. Those of Cliilina, 

 which inhabit the clear running streams of South America, are almost 

 exactly like Auricula, which the animals of this family greatly resem- 

 ble. The shell of Amphipejplea is transparent and swollen ; and is 

 nearly covered by the sides of the mantle. 



Family Planorbid^:. 



The animals of this family differ from the Limnseids in having sharp, 

 pointed tentacles. The shape of the shells is, extremely variable. In 

 the first group they are flat, in the second pointed, and in the third 

 limpet-shaped. 



Planorbis has a spiral shell with the whirls inclosing each other on 

 the same plane. It lives in a reversed position. The whirls are flat 

 and numerous in most of the European species ; generally few and 

 swollen in the American. Monstrosities are found, perpetuating them- 

 selves in particular ponds, with the spire elevated. The teeth closely 

 resemble those of Auricula-. One of the minute British species has 

 no fewer than six thousand of them. Some species emit a purple fluid 

 when disturbed. In Segmentina the whirls are divided across at reg- 

 ular intervals, by septa with toothed openings for the passage of the 

 animal. So little was known of its true relations in earlier times, 

 that the British species was called the " Freshwater Nautilus/' 



The Physa tribe have shells looking like reversed Limnceas. In 

 the typical species they are enveloped, as in Amphipeplea, by the 

 fringed sides of the mantle. In the beautiful group Aplexa, the shells 

 are glassy, with raised spires, and the mantle margin is plain and not 

 flapped. Physopsis is a south African form, like a reversed Achati- 



