Inferiorly, the eight interseptal compart ments communicate freely with the great ca vity (d), situated beneath the stomach, the membranous septa becoming gradually conti nuous, with eight longitudinal folds ( j') that project into its interior. Just at the point where each of the membranous septa ceases to be continuous with the walls of the sto mach and becomes free by its inner margin, may be observed a filiform flexuous organ (h), the nature of which appears to be glandular.
The common polypary from which these polypes issue is composed, as was stated above, of two distinct portions ( fig. 30, b, e), The superior soft portion is found by dis section to be made up of an assemblage of longitudinal membranous tubes placed parallel to each other, and so closely conjoined that it is difficult to separate them, and in fact the hard base of the polypary is nothing more than a continuation of these tubes, slightly altered in their structure : those situated near the centre of the stem are only' distin guishable by a slight thickening of their walls, but those near its circumference acquire a much harder consistence, their parietes being encrusted with multitudes of brown-coloured fusiform spicula, vvhich appear to be composed of a cartilaginous substance and of carbonate of lime. These spicula are arranged in a longitudinal direction. and confer upon this part of the polypary its solidity and peculiar aspect. Near the circumference of the poly pary many of the tubes seem to be obliterated • by pressure of the contiguous parts. On tracing the tubular structure downwards towards the base, each tube gradually disap pears, either by becoming obliterated, or by anastomosing with the surrounding ones, whilst superiorly it is found to be continuous with the abdominal parietes of a poly pe, the sheath of which it forms when it is in a state of contraction.
The tubes thus united into fasciculi are evidently analogous to the cavities into which the polypes of Alcyons, Corals, &c., withdraw. These cavities are generally called "poly pi ferous cells," and some writers consider them as being species of cases or envelopes more or less distinct from the animals themselves ; but in the Zoophyte we are speaking of a superficial examination is sufficient to con vince any one that these cells are in reality continuations of the bodies of the polypes themselves. The tubes which form the trunk ( fig. 30, c), are in all respects similar to the free portion of the animal which is situated beneath its alimentary canal, and no line of organic separation can be traced di viding one from the other. It is not, there
fore, into polypiferous cells that these little animals retire in the manner of Serpulx or Dentalia, but they. recede into themselves by a kind of invatination of their own bodies, the polypary, which seems to contain them, being simply- a mass formed by an assemblage of the basal portion of all the aggregated zoophytes.
When the polypes extend themselves, their mouths may frequently be seen to open and admit the surrounding water. This fluid, together with the alimentary materials sus pended in it, penetrate into the digestive sac culus, and afterwards pass into the great ab dominal cavity, whence they are conveyed even into the tentacula by the eight canals placed around the alimentary tube. It thus results that the thin and diversely folded membrane, of which the bodies of these animals are formed, is everywhere bathed, both within and without, with the materials for respiration, and that all its internal surface receives the contact of the alimentary sub stances after their elaboration in the digestive sacculus. M. Milne Edwards likewise thought he perceived something like a circulation in canals contained in the parietes of the body; but of this he was uncertain.
Nutrition of Atcyonide. — It is very gene rally admitted that in the case of these ag gregated zoophytes the nutritious materials taken by one of the animals is shared with the neighbouring polypes; and this fact M. Milne Edwards has established beyond the possibility of doubt, as well as the manner in which it is effected. In an expanded Alcyo nide he introduced, by means of a fine pointed glass tube, a coloured fluid into the abdominal cavity of one of the polypes, and immediately the injected material difffised itself, not only throughout the tubiform body of the indivi dual so treated, but passed at the same time into the neighbouring polypes. The passages by which this communication is established are easy to discover, by making a lorwitudinal section of the body of the Alcyonide. It is then seen that some of these animals, whose tube-like bodies are prolonged deeply into the common mass, there terminate by cuts de sacs, whilst others are not continued beyond the point where they join their congeners ; and in this case their bodies are found to be continuous with that of a larger polype, the basal portion of which descends lower down ( fig. 32). The abdominal cavities of these animals are thus united, so as to constitute a kind of branched tube, possessed of as many heads and mouths as there are polypes de rived from it.