The Siphonostomata form a much smaller section than the last, and its members are distinguished mainly by the presence of a true siphon, formed by the prolongation of the mantle margin, and serving to convey water into the gill chamber. In all these the shell is spiral, usually without an umbilical opening, and the margin of the mouth is prolonged into a canal or distinctly notched. The operculum is horny, and lamellar or imbricated. The animal has a retractile proboscis, and the eyes or eye-pedicels are joined to the tentacles. All the species of this division are marine.
Fig. 178.—Cypræa (Trivia) europæa
We will first take the family Cypræidæ, which contains the familiar Cowries, these forming the lowest group of the division. An examination of the shells may at first seem rather puzzling, for the spire is concealed, and the whole is convoluted in such a manner as to make the mouth long and narrow, with a channel at either end. The outer lip is also thickened and bent inward, and there is no operculum.
The animal itself is particularly interesting, for, as it creeps along on its broad foot, abruptly shortened in the front, the mantle lobes bend over the top, meeting along the middle line, where they are usually fringed with little tentacle-like processes; and, as a result, the whole shell is beautifully enamelled on the outer surface. In all the Cowries the central teeth are single, and the laterals are arranged either in twos or threes.
Perhaps the commonest representative of this family is the pretty little Cypræa (Trivia) europæa (Plate V.), the shells of which are sometimes washed up in large numbers on sandy beaches. The animal lives mainly below low-water level, but it may often be found in the larger rock pools, creeping rapidly over the tangles, and may be easily secured with the aid of a net.
In the same family we have the little Erato (Marginella) lævis, the white shell of which is minutely furrowed along the lips; and also Ovulum patulum (Calpurna patula), so called on account of its fancied resemblance to a poached egg.
We have also several species of Cone shells (family Conidæ) on our coasts, readily recognised by their form, which is a cone, with a long, narrow aperture, partially closed by a minute operculum. As in the last family, the foot is abruptly shortened in front. The head is very prominent, with eyes situated on the tentacles. There are two gills, and the teeth are arranged in pairs.