On Molecular and Microscopic Science, Volume 2 (of 2)

by Mary Somerville

Fig. 124. Polype of Alcyonidium elegans.

The narrow slit of the mouth opens into the stomach, which is a flat, short sac hanging down in the central cavity of the polype’s body, with an orifice at its lower end. The stomach is fixed to the internal walls of the body by eight vertical folds forming so many longitudinal chambers open at their lower extremity. The whole of the surface of the interior, the walls, the stomach, and the septa or divisions, are covered with fine cilia, by whose vibrations constant currents are maintained in the water which bathes every part of the cavity freely entering at the mouth. The polypes are carnivorous, living upon infusoria and minute particles of animal matter floating on the water, which they seize with their mouth, or arrest with their flexible and contractile tentacles. The food is digested by the solvent juices in the stomach, and the refuse is ejected at the mouth.

The eggs of these polypes are formed and fertilized among the vertical folds adjacent to the stomach. When hatched, the larvæ pass through the stomach and come out at the mouth as active ciliated creatures, so like eggs that the Alcyon zoophytes were believed to be oviparous. However, in some of the genera they are discharged through pores between the bases of the tentacles.

Fig. 125. Spicula of Alcyonium digitatum.

The Alcyon polypes have multitudes of needle-like spicules, rough with projecting knots. They are collected into triangular groups at the foot of each tentacle; the central and largest point runs up into the tentacle. Towards the lower end of the polype, spicules again occur scattered through the skin and crowded into groups, as in [fig. 125]. These, however, form short thick cylinders, each end being dilated into a star of five or six short branches. The spicules always contain an organic base hardened by carbonate of lime, for when Dr. Carpenter dissolved the lime with dilute acid, a gelatinous substance remained, which had the form of the spicules. [Fig. 125] shows those of the Alcyonium digitatum, or Dead Man’s Fingers, generally assumed as the type of this numerous order, which contains sixteen genera and many species, differing much in form but connected by a similarity of digitate structure.

The Alcyonium digitatum, when torn from the rock to which these animals are attached, shrinks into a cream-coloured fleshy mass of somewhat solid texture, rough and hard to the touch, and studded all over with hollow depressions or pits. When put into sea-water, these lumps, from the size of a pea and upwards, expand, become semi-transparent, and from each depression a polype protrudes its beautifully symmetrical eight-petalled blossom. Their tentacles are short, broad, and prehensile; and the slender pinnæ, which fringe their edges arching outwards, are seen with a high magnifying power to be rough with prickly rings, discovered by Mr. Gosse to be accumulations of thread-cells with their darts.

These Alcyons, when expanded, are about an inch-and-a-half high and two-thirds of an inch thick, but individuals are met with two or three times as large, and much divided into blunt finger-like lobes. The sarcode mass of these compound animals is channelled like a sponge, by branching canals, the orifices of which open into the stomachs of the polypes; and, by bringing them into communication with each other, unite the whole into one compound animal, which is maintained by the food caught and digested by each individual polype. Currents of sea-water mixed with the nutritious juices are made to circulate through the branching canals by the vibrations of cilia with which they are lined; they flow round the stomachs of the polypes, supply their juices with oxygen, and carry off the carbonic acid gas and refuse of the food. In this case, as in many others, the cilia may be regarded as respiratory organs.

The unarmed Alcyons are generally thick, short, and rough; some form a crust on rocks from whence lobes rise. With the exception of the Xenia, a tropical species, the polypes of the unarmed Alcyons can retreat within their polypary, so as to be entirely or partially out of sight.