The Sea Shore

by William S. Furneaux

Fig. 162.—Nudibranchs

1. Dendronotus arborescens. 2. Tritonia plebeia. 3. Triopa claviger. 4. Ægirus punctilucens

In fig. 162 we represent four species. Two of these—Triopa claviger and Ægirus punctilucens—belong to the family Doridæ, the members of which are popularly known as Sea Lemons, and are distinguished by the presence of plume-like gills situated on the middle of the back. Another family (Tritoniadæ), characterised by the arrangement of the gills along the sides of the back, and by tentacles that can be retracted into sheaths, is represented by Tritonia plebeia and Dendronotus arborescens in the same figure, and by Doto coronata in fig. 161. The family Æolidæ also have their gills arranged along the sides of the back, but they differ from the last in that their tentacles are not retractile. They include the two species numbered 3 and 4 on fig. 161. The remaining one on fig. 161—Elysia viridis—is a member of the family Phillirhoidæ, characterised by a pair of tentacles on the dorsal side of the head and by the foot being either very narrow or absent, the latter feature denoting that the animals are not adapted for creeping on the bottom. In fact, several of the species of this family swim freely by means of flattened tails.

The Tectibranchs are similar in general structure, but are very different in appearance, inasmuch as the gills, so prominent in the last division, are here covered by the mantle, or by the shell, which is often well developed. The latter is very variable in form, being of a globular, twisted, spiral, or other shape, but is sometimes absent in the adult. In fig. 163 we give a few examples of the shells of British species; and one (Bulla hydatis) is shown on [Plate V].

Fig. 163.—Shells of Tectibranchs

We now pass on to the largest and last order of gasteropods—the Prosobranchiata—so called because the gills are situated in front of the heart. This group is an important one to the sea-side naturalist, since it contains nearly all the univalve molluscs that are common between the tide-marks of our shores, as well as some abundant species that are protected by a shell of several distinct parts. In nearly all of them the abdomen is well developed, and the shell is sufficiently large to cover the whole animal when the latter is retracted; and the gills, which are either pectinated (comb-shaped) or plumed, are lodged in the chamber formed over the head of the animal by the mantle.

The order is often divided into two sections—the Holostomata or Sea Snails, in which the margin of the aperture of the shell is entire, and the Siphonostomata, in which the margin of the mantle is prolonged into a siphon by which water passes into the gill chamber. This division does not seem to be very satisfactory, as the sections are not separated by very prominent natural characteristics, but it becomes convenient on account of the great extent of the order.

In the Holostomata the shell is either spiral, conical, tubular, or composed of several valves, and the spiral forms are usually closed by a horny or shelly operculum of the spiral kind. The head is provided with a proboscis that is generally non-retractile, and the gills usually extend obliquely across the back, or are attached to the right side behind the head.