The Pectinibranchs are known as violet sea-snails, Ianthinidæ and Scalariidæ. The radula consists of numerous rows of pointed teeth arranged in cross series, forming an angle in the middle. There is no central or rachidian tooth, and they have thin trochiform shells adapted for a pelagic life. They are mostly of a violet colour, from which they derive their name, the colour being more vivid on the underside, which is turned up towards the light when the animal is swimming near the surface of the sea ([Fig. 370]).
Fig. 370.—Ianthinia, Violet Sea-snail.—(Warne.)
The bubble b, drawn somewhat too large, is about to be joined to the anterior end of the float; c. Shell; l. Float; p. Foot; t. Head.
The most interesting feature in connection with these oceanic snails is the curious float which they construct to support their egg-capsules. It is a gelatinous raft, in fact, enclosing air-bubbles, which is attached to the foot, the egg capsules being suspended from its under-surface. They are unable to sink so long as they are in connection with their floats, and are therefore often cast on shore during storms, and furnish an endless series of microscopic specimens. The violet snails feed on various kinds of jelly-fish, and occur in shoals.
Pond Snails.—The three families, Limnœidæ, Physidæ, and Chilinidæ, form a special group of the pulminate, sessile-eyed fresh-water snails. The larger family of these belongs to the genus Limnœa, having a compressed and triangular head with two tentacles and eyes placed at their inner base. They are prolific and gregarious, and their ova are enclosed in transparent gelatinous capsules, deposited in continuous series, and firmly glued to submerged stems and leaves of aquatic plants. L. stagnalis is common in all ponds, marshes and slow-running rivers of Great Britain.
Fig. 371.—Ova and young of Limnæus stagnalis.
One of the species, L. trancatula, is the host of the liver-fluke so fatal to sheep. The fluke parasite passes one stage of its existence in the intestine of the pond snail.
Each ova-sac of Limnœa contains from fifty to sixty ova (represented in [Fig. 371], at a). If examined with a low power soon after the eggs are deposited, they appear to consist simply of a pellucid protoplasmic substance. In about twenty-four hours a very minute yellowish spot, the nucleus, is discovered near the cell-wall. In another twenty-four hours the nucleus referred to is seen to have assumed a somewhat deeper colour and to contain within it a minute spot—a nucleolus.