Fig. 174. Whelk.
Fig. 175. The Crowned Eolis.
In the group of the Eolidæ, the gills are like leafless trees in most genera, but in the principal genus Eolis, they are long, spindle-shaped, sharp-pointed papillæ, arranged in transverse rows or clusters along the sides of the back, leaving a space between them, as in [fig. 175]. They are covered with long cilia, whose vibrations send a perpetual current of sea-water along each of them, the respiration is aided by vibrating cilia, scattered almost over the whole body, and the circulation of the blood is very simple.
Fig. 176. Tongue-teeth of Eolis coronata.
The Eolis has a head prolonged into a pair of tentacles which are active and as sensitive as antennæ. Another pair on the back have ten or twelve narrow plates twisted in a spiral round them; the eyes are at the base of these horns. The mouth contains horny jaws and a spiny tongue like a mere strap covered by numerous transverse plates armed with recurved spines not more than a sixth part the thickness of a human hair. [Fig. 176] represents the tongue and some of the spines greatly magnified. The mouth leads into a short and large membranous stomach, from each side of which branches are sent off, from whence long canals traverse the papillæ longitudinally, and perform the part of a liver. In many species these tubes are brilliantly coloured, but none are more beautiful than those in the Eolis coronata, which is found under stones, like a mass of jelly, not larger than a pea, at low spring tides, on our own coasts. When put into sea-water it expands till it is about an inch long ([fig. 175]). It is then pellucid, tinged with pink, and the central tubes in its numerous papillæ are of a rich crimson hue, their surface reflects a metallic blue, and their tips are opaque white; as the animal keeps its papillæ in constant motion the effect is very pretty.
The Eolis coronata, like all its congeners, has a stinging apparatus, consisting of an oblong bag, full of thread cells, placed at the extremity of each papilla, from whence darts can be ejected through an aperture in the tip. The whole of the Eolididæ are carnivorous, fierce, and voracious, setting up their papillæ like the quills of a porcupine when they seize their prey; they tear off the papillæ of their weaker brethren, and even devour their own spawn, though their chief food consists of zoophytes.
The Pteropoda, or wing-footed mollusca, are very small; they are incapable of crawling or fixing themselves to solid objects, but they are furnished with two fins like the wings of a butterfly, with which they float or row themselves about in the ocean, far from land in vast multitudes. The shell of the typical species Hyalæa (A, [fig. 177]), which resembles the thinnest transparent glass, consists of two valves, one, which is placed on the front of the animal, is long, flat, and ends in three points; the other valve, which is applied to the back, is short and convex, and in the lateral fissure between the two, the mantle is protruded. The head and fins project from an opening at the top of the shell. The fins, which are formed of muscular fibre, are fixed on a short thick neck, with the mouth lying between them, containing a tongue crossed by rows of long reversed teeth. The head has no tentacles, and the animal appears to be blind, but it has an auditory vesicle lined with cilia, which keeps a few otolites in motion. This little animal is highly organized; it has a gullet, a kind of crop and gizzard, a liver, a respiratory tube, a heart, a circulating and nervous system, which enables it to swim with a flapping motion of its fins.