This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
When examined under the microscope, the granules extracted from their investment are found, in the male, to contain immense numbers of caudate and very active spermatozooids, - while those of the female are real ova, of spherical shape, and furnished with a distinct yelk and germinal vesicle.
(171). It is during the months of August and September that the generative system of our native species is in full activity and development. The ovum, it would seem, when arrived at maturity, breaks loose from the ovarian nidus; and as the number of males in a given locality is pretty nearly the same as that of the females, and they are always more or less found in company notwithstanding their sedentary habits, the eggs of the female would seem to be impregnated by the seminal fluid of the other sex, diffused through the surrounding water.
(172). In Cerianthus membranaceus, however, a complete hermaphroditism exists, all the convolutions of the reproductive ribands being equally supplied with both the male and female elements of generation, in the shape of minute capsules, some of which enclose an ovule, whilst the others are filled with spermatozoa. These two kinds of capsules are scattered promiscuously, without any regular order, but always so that the ovigerous and spermatogenous organs are closely in contact. The ovules on quitting their ovarian nidus fall into the general perivisceral cavity, where they may be found of a spherical or oblong shape, and each presenting a distinct Purkinjean vesicle. The act of fecundation in this case appears to take place in the ovarian laminae, and probably by the rupture of the delicate walls which circumscribe the contents of the male and female capsules. The eggs found in the ovaria are round and of a yellow colour, resembling minute grains of sand, and densely ciliated. There exists a considerable opening in the base of the stomach, whereby a free communication is established between the interseptal spaces and the general abdominal reservoir, through which the young Actiniae are expelled in a very advanced state of development into the stomach, and from thence pass through the mouth into the surrounding water.
The smallest germs are semi-opake spherical bodies, while the more advanced present every gradation of form from the simple sphere up to the complete tentaculate polyp. The largest are about the size of peas. On section they present appearances similar to those exhibited in the annexed diagrams (fig. 37), intended to illustrate the manner in which the morphological changes are brought about and the several special organs of the Actinia unfolded. The figures may be thus explained: -
1. Outline of the mature embryonic corpuscle after the disappearance of the cilia with which, at an earlier stage, it is furnished.
2, 3, 4. Primary involution of the integumentary membrane. 5, 6. Re-induplication of the external membrane and formation of a stomachal cavity. In the two latter figures may likewise be seen the commencement of the tentacula (a) and ovarian septa (6), which are all formed by the same process of involution*.
(173). The perigastric spaces of the Actiniae enclose, in addition to the reproductive apparatus, abundant convolutions of very remarkable filiform organs of great length and tenuity (fig. 38), concerning the nature of which much diversity of opinion still exists. These convoluted threads have, in fact, no communication whatever with the generative organs; they seem to consist of long, semi-capillary caecal tubes, attached by a short mesentery to the lower margin of the perigastric septa; and from each of them a cord, slightly larger than the filament from which it is derived, may be traced upwards along the corresponding septum as far as a little above the inferior termination of the alimentary cavity, and then running along the external wall of the stomach as far as the pyloric opening. The structure of the terminal portion as well as that of the convoluted filament is tubular, and their office apparently important in the economy of the animal: M. de Blainville regards them, with some probability, as representing the biliary system.
Fig. 38. Convoluted filiform organs of Actinia.
* " Observations on the Anatomy of Actinia," by T. Spencer Cobbold, M.D., Annals of Nat. Hist., Feb. 1853.
(174). The Abbe Dicquemare relates several curious experiments on the multiplication of these animals by mechanical division. When transversely divided, the upper portion still stretched out its tentacles in search of food, which, on being swallowed, sometimes passed through its mutilated body, but was occasionally retained and digested. In about two months, tentacles grew from the cut extremity of the other portion, and this soon afterwards began to seize prey. By similar sections he even succeeded in making an animal with a mouth at each end.
(175). After the account above given of the general structure of the Actinia, the mechanism whereby the tentacula are expanded and withdrawn will be easily understood: these do not, like the horns of a snail, become inverted and rolled up within the body, but owe their different states of extension entirely to the forcible injection of water into their interior. We have seen already that the cavity of each tubular arm communicates freely with the space intervening between the stomach and the external integument - a space which, at the will of the animal, is filled with sea-water drawn through the orifices placed at the extremity of each arm: when these minute orifices are closed, and the body of the creature contracted, the water, being violently forced into the tentacula, distends and erects them, as when watching for prey; and, on the other hand, when emptied of the fluid thus injected, they shrink and collapse. This circumstance, so easily seen in the Actiniae, will likewise enable us to account for similar phenomena observable in other polyps, the internal economy of which is by no means so conspicuous.