This condition of their nutritive system arises from the mode of their development b y gemmiparous reproduction, which takes place as follows. A tubercle makes its appearance upon the surface of the body of an adult po lype, which seems at first to be only a little ccecal appendaae developed from its parietes, its extremity being without any opening and the cavity in its interior communicating freely with the abdominal cavity of the individual from which it was developed. Shortly, how ever, as its development proceeds, a mouth and its surrounding tentacula make their ap pearance, an alimentary cavity becomes ap parent, and the newly formed animal becomes, both in shape and size, exactly like the indi vidual from which it sprouted.
This mode of reproduction, Milne Ed wards remarks, does not occur at any point of the tegumentary surface. The reproduc tive gemmm are only formed along the course of the membranous lamellae already men tioned, and the inferior opening of the body of the new polype is Always so situated as to intercept one of the longitudinal folds in the abdominal cavity of the parent animal.
But the mode of reproduction by gemmm is not the only one by which the Alcyonides are multiplied. They produce also ova, or gemmules, by means of which their sedentary race may be disseminated; and it is remark able that the same parts which give birth to the gemmx above described perform likewise the office of ovaries. The ova are, in fact, developed in the substance of the longitudinal membranous folds from which the gemmm sprout. As they grow larger they project internally, and soon become pedunculated ; at last, when mature, they detach themselves from the ovigerous fold and fall into the abdominal cavity, whence an issue is afforded to them through the mouth of the polype. No ovule is ever developed from the parietes of the abdominal cavity intervening between the longitudinal folds ; and hence there can be but little doubt that these lamellm represent the ovaria of the animal.
On seeing the same organ producing some times buds, or geninme, and sometimes ova, Milne Edwards was led to inquire into the cause of this difference in the mode of repro duction, which he conceives to be of a me chanical nature. In those parts of the polype which are not yet imprisoned in the growing mass of the polypary, reproduction is gene rally effected by the development of external buds, while towards the base of the polypary, where the constituent zoophytes are inti mately united together by their outer surface and are surrounded by a sort of sheath, no external buds are formed, but the ovules make their escape into the internal cavity of their parent. Hence the distinguished zoolo
gist, whose memoir we quote, is led to infer that, on the one hand, the mechanical ob stacles to be encountered, and on the other the excitement occasioned by the contact of the surrounding element, determine this dif ference of procedure, and that the membrane which performs the functions of an ovary produces indifferently either ova or gemnix, according as it finds less resistance or is more stimulated on the inside or the outside of the abdominal walls.
From the above details it becomes easy to explain how a single polype, by its repro ductive powers, can form the complicated mass of the compound polypary of the Alcyo nide, as well as the nieans whereby an organic continuity is established between all the indi viduals of the coinmunity; also how the abdo minal cavity of the primitive individual be comes common to all the young ones that sprout froin it ; in short, how the little beings, thus united together, rather resemble a single multiple aniinal than an assemblage of distinct individuals. But with the advance of age this intimate union gradually diminishes. The communication between the abdominal cavi ties of the different polypes whose basal por tions reach as far as the foot of the polypary is first of all interrupted by the ova, with which the lower part of these cavities becomes filled (fig.33, f); and subsequently, by the pressure of the surrounding parts, the walls become confused, and all communication be tween the polwe whose abdominal tube is thus obliterated and the polype from which it sprung is intercepted.
The polypary, instead of resembling a tree, all the flowers of which hold together and communicate by common parts, may now be compared to a bouquet made by cutting off the more or less branched twigs of a plant and collecting them in a bundle. The dif ferent groups of polypes united in tbe same polypary become thus independent of the neighbouring groups, and, as ma,y readily be conceived, in time each polype can become individualized.* The filiform organs (fig. 41, k), situated be low the digestive cavity, are evidently not ovaria, as they have been considered to be by many authors, seeing that the ova are formed elsewhere ; neither does M. Milne Edwards consider that they can be seminiferous organs, but is inclined to regard them as hepatic vessels.