In the genus Aleyonium the zoophyte is composed of two principal portions. The common central mass is of a coriaceous tex ture, porous, and somewhat like cork, being formed of a dense substance, which, when cut into pieces, feels gritty under the knife, owing to the quantity of earthy spicula diffused through its mass. Externally it consists of a reddish granular substance, in which the polype cells are excavated, but internally it is of a grey colour, and permeated by numerous tubes that descend towards the base of the zoophyte, and frequently run into each otber. These canals are filled with a gelatinous fluid, and lined with a red material prolonged from the external layer.
The polypes which stud the surface are as fine as hairs ; but still, with the aid of the microscope, it is not difficult to distinguish the mouth, the vesicular stomach, the muscu lar envelope of the animal, the ovary, and the glandular organs which depend from the base of the stomach into the abdominal cavity of the polype. Its whole structure has been well described by Spix, and subsequently more in detail by Milne Edwards in the paper above referred to. The following is the re sult of Spix's observations.
" The mouth is a small rounded aperture, vvhich is very dilatable, and communicates immediately with the stomach. The mouth is surrounded by eight tentacles, having a papillary surface, and they appear to contain internally a multitude of little air bubbles. They are very sensible, for as soon as they are touched they retract, and the animal re tires into its cell.
" The polype is retained in its domicile by a muscular membrane, which is very distinct from the walls of the stomach, and is almost cylindrical ; it descends from around the mouth, and is fixed to the edges of the cell ; it appears to form the tentacles and the sto mach, as in Actinia. The contraction and extension of the polype is effected by this membrane." For many days durina which Spix watched these polypes he observed little globular bodies to ascend from beneath the stomach and issue at the mouth. By pressing gently he saw them glide as by a little orifice into the sto mach, and by the same proceeding he suc ceeded in pushing them under it.
Having raised the muscular membrane at the point where it is fixed to the polype, he perceived at the bottom of the cell, and be neath the stomach, seven or eight globules contained in a bent canal (ovary), placed in a row. They gave to the canal the appearance of a row of vesicles. The globules are round ; those which are most developed red, each enclosing a multitude of ova.
" When the animal is drawn out of its shell, by tearing the muscular membrane, the ovary detaches itself from the stomach and remains at the bottom of the cell. But there is an other grey body like a tail, which follows the stomach, and is attached to it opposite to the ovary. This body is round, very thin, and so
slender that it does not fill the tube in which it is placed ; it is therefore difficult to imagine that it descends to the base of the zoophyte to unite with the rest." The above account, it will be perceived, agrees very closely, as far as it goes, with Milne Edwards's description of the anatomy of the Alcyonidium ; but the last mentioned naturalist has prosecuted the investigation of these zoophytes still more minutely.
In the Alcyons, properly so called, the vascular system is very distinctly developed, and in Akyoniunz stellatunz, more especially, M. Milne Edwards was able to study it with faci lity. In this species he was enabled to detect upon the parietes of the abdominal cavity of the polype a variable number of minute aper tures irregularly dispersed, which are in im mediate communication with a system of ca .pillary canals that traverses in all directions the spongy portion of the polypary formed by the external tunic of its component animals. For in this species it is very easily seen that while the internal tunic lines the abdominal cavity of the polype, the external layer, instead of being confounded with the former, as in the protractile portion of the animal, becomes perfectly distinct from it at the point where it begins to enter into the composition of the polypary, at which its thickness becomes con siderably augmented, its texture spungoid, and in its substance are deposited a number of irregular crystals, composed of carbonate of lime mixed with a little colouring matter. In the tegumentary mass thus formed, the vascular canals ramify, anastomosing freely among themselves, so as to constitute a vas cular network. These vessels are formed of very attenuated membrane of a yellowish colour, which is continuous with the internal tunic of the polypes, and is perfectly distin guishable from the dense tissue with which it is surrounded. The distribution of these canals is best displayed by cutting a thin slice of the mass of the Alcyon and removing the crystals with which it is filled by immersion in some dilute acid ; it is then seen that the canals are most numerous and of the largest size towards the extremities of the branches of the polypary, and that they establish fre cl uent communications between the abdominal cavities of the different polypes. This organi zation evidently establishes a very intimate connection between the different polypes of the Alcyon. The fluids with which their bodies are filled must thus necessarily circulate in the entire mass of the polypary, and if each of the polypes has, on the one hand, an indi vidual sensibility, and a distinct digestive cavity on the other, there is a vascular system common to them all.