(120). Although essentially similar in their habits, the compound polyps present various modifications of structure, which naturally causes them to be grouped in distinct families. Sometimes the central common mass is entirely soft and gelatinous, its surface being covered with minute cells in which the polyps are lodged: such are the Aleyonidae. Sometimes the common body secretes large quantities of calcareous matter in the same manner as the Fungia, which, being deposited in its interior, forms arborescent masses, presenting upon their surface multitudes of cells, generally distinguishable after the removal of the outer crust, in each of which when alive a polyp was lodged: these form the family of Madreporidm. The central axis is not unfrequently quite solid and smooth upon the surface, offering no cells for the lodgment of the polyps - being either composed of hard and dense calcareous substance, or else flexible and horny in its texture: such are the Corallidce, or family of corals, properly so called.

The internal central axis is moreover, in another family, composed of several pieces united together by the living crust that secretes them; and being free and unattached, such forms are probably able to change their position at pleasure: these constitute the family of Pennatulldae., or swimming polyps. They are, however, all merely modifications of the same general type of structure, although differing in certain minor points of their organization, so as to render an examination of each form needful for our purpose.

(121). Alcyonidae

This family includes several genera, known by the names of Alcyonium, Lobidaria, Cydonium, etc, being characterized by having no solid axis developed in the interior of the common body. The Cydonium Mulleri (fig. 24) will give the reader a good idea of the general appearance of one of these compound animals. The central mass, or polypary, is entirely soft, being of a gelatinous, or, rather, subcartilaginous texture. Upon cutting into it, it is found to be intersected by tough fibrous bands, and not unfrequently contains calcareous spicula dispersed through its substance: no muscular fibre or nervous matter has ever been detected in its composition, and its interior is permeated by numerous wide canals variously disposed.

(122). Few objects exhibit to the naturalist a more beautiful spectacle than the compound animals of which we are speaking. When found upon the shore contracted and deformed, it would be difficult to imagine that they were really organized beings, much less possessed of any elaborate conformation; yet on placing one of them in a tumbler of sea-water and watching it attentively, its true nature is gradually revealed: the central mass expands in all directions, exhibiting the cells upon its surface, from which in time the countless flower-like polyps are protruded, which, stretching out their arms in all directions, wait for the approach of prey. A scene like this naturally leads us to inquire concerning some points of physiology connected with their economy; and several questions obtrude themselves upon us, which, as they are applicable to the whole group of compound polyps, may be well discussed in this place.

(123). That there is a community of nutrition - or, in other words, that food taken and digested by the individual polyps is appropriated to the support of the general body - is generally admitted; but is there a community of sensation, so as to render the entire mass one animal, capable of consentaneous movements? or is each polyp independent of the rest in its sensations and actions? Upon this there are different opinions, - some regarding the whole as a single animal, each part being in communication with the rest, and thus participating in the feelings and movements of the others, whilst some consider every polyp as a distinct creature, independent of the rest. The solution of this problem is a matter of some difficulty; but there are several facts which may in some measure enlighten us upon the subject. From the absolute want of nervous filaments which might bring into communication distant points of the body, we might, theoretically, deny the possibility of any combination of actions; and experiment teaches us that the assumption is correct.

Cydonium Mulleri.

Fig. 24. Cydonium Mulleri.

(124). If, when one of these animals is fully expanded, transparent, and soft, any point of its surface be rudely touched, the whole body does not immediately shrink, but only the point where the irritation was applied appears to feel the impression; this part shortly becomes more dense, opake, and a depression is seen gradually to appear. If the shock be severe, and extensively diffused over the body, the contraction slowly extends to the whole mass: the most violent local injury, indeed, seems to be totally unperceived at remote parts of the body; whilst a general shock, such as striking the vessel which contains the expanded polyp, produces a simultaneous contraction of the whole*. The polyps, however, exhibit much greater irritability than, and their movements, from their rapidity, form a striking contrast to the languid contractions of the central portion: but that they have a community of life appears improbable; they seem to act quite independently of each other: when one is touched, and suddenly retracts itself within its cell, it is true that those in the vicinity will likewise not unfrequently retire; but this circumstance may be accounted for by the sudden movement of their neighbour; for as the polyps are closely contiguous to each other, there is no cause for urging a community of substance to explain it1.

(125). It has been observed by Milne-Edwards 2, in Alcyonidium (fig. 25) - a genus of Alcyonian zoophytes remarkable from the circumstance that its polypary, or common body, consists of two portions of very different consistence, the upper part or trunk (c) being quite soft and flexible, while the lower portion or foot (6), by which it is attached, is of a hard and solid texture, - that although under ordinary circumstances the movements of the individual polyps are quite independent of the rest, a simultaneous contraction of the whole may be excited by irritating the common trunk, and that to such an extent that, if the stimulation be excessive, the whole of the soft portion of the polypary is retracted into a coriaceous sheath afforded by the foot.