The most probable appears to be that of Dr. Sharpey, who suggests that, should the fluid which distends the feet, and the vessels connected with them, be indeed sea-water, it may be introduced, and perhaps again discharged, through the pores of the disk, by means of the calcareous tube, which will thus serve as a sort of filter to exclude impurities.

(477). Apparently with a view to ensure a continual circulation of aerated fluids through all parts of the system, the entire surface of the membrane that lines the shell, as well as that which forms the external tunic of the digestive organs, has been found to be covered with multitudes of minute cilia, destined by their ceaseless action to produce currents passing over the vascular membranes, and thus to keep up a perpetual supply of oxygenated water to every part*. But it is not only on the peritoneal surfaces that the existence of cilia has been detected; they are found to be extensively distributed over the external surface of the body, within the cavities of the tubular feet, and even over the whole internal lining of the stomach and caeca.

* Cyclopaedia of Anatomy and Physiology, art. " Echinodermata".

(478). "In Asterias rubens," says Dr. Williams, "it can be distinctly demonstrated that no open perforations exist in any part of the integumentary parietes. The membranous processes communicating with the. visceral cavity can be proved, by injection, to be caecal at their distal extremities. . It is easy to repeat and confirm the observation of Dr. Sharpey, that the corpuscles of the visceral fluid advance to the distal end of these processes, and then return, under the impulse of ciliary agency. Nevertheless, although an injection so thick as size will not escape through these membranous processes, a thinner fluid, such as coloured water, will slowly ooze through; it is not, therefore, improbable that an interchange of the fluids may to some extent occur through endosmose. The microscope renders it certain that the hollow membranous processes filled by the fluid of the visceral cavity, in Asterias, bear in their parietes no trace of true blood-vessels: they are lined within and without by vibratile epithelium, and composed only of interlacing elastic fibres; and consequently their only office seems to be that of exposing the chylaqueous fluid to the renovating influence of the surrounding medium. In Asterias, this fluid approaches simple sea-water closely in its physical properties.

It is, however, in reality a dilute, albuminous, opalescent solution. It is charged scantily with imperfectly formed corpuscles, always the same in the same species; and the proposition may now be confidently affirmed, that in the Echinoder-mata the chylaqueous fluid (i. e. the contents of the visceral cavity) is itself first aerated, and that by means of a machinery of soft parts it then aerates the blood proper".

(479). The organs belonging to the reproductive system in the Aste-ridae exhibit the greatest possible simplicity of structure. The ovaria (fig. 88,1, f) are slender caeca, arranged in bunches around the oesophagus, two distinct groups being lodged at the origin of each ray. In Asterias aurantiaca (fig. 90), the excretory ducts are not easily seen; but in the Twelve-rayed Star-fish, especially if examined when these organs are in a gravid state, each ovary may be observed to communicate externally by a wide aperture that perforates the osseous circle encompassing the mouth.

(480). The generative organs of the male individuals exactly resemble those of the female, and are only distinguishable by the presence of spermatozoa in their interior. The process of reproduction 1 usually occurs during the spring months, at which period the ovaria of the females are found distended with eggs, wherein the vesicles of Purkinje and of Wagner are distinctly recognizable. These ova are found in the ovaria in different stages of development, and are laid in successive batches at different intervals.

* See the article " Cilia," by Dr. Sharpey, in the ' Cyclopaedia of Anatomy and Physiology.'

1 Memoire sur le Developpement des Asteries, par M. Sars, Ann. des Sc.Nat. 1844.

(481). The newly-laid ova consist of a chorion enclosing the vitellus and a small quantity of albumen; but the vitellus soon undergoes the usual process of segmentation, whereby it is broken up into a granular mass (fig. 91, 4, 5, 6,7, 8.) When first deposited, the ova of the Starfishes are not at once abandoned by the parent animals, but are retained in a kind of cavity formed by incurving the body and rays of the mother until they form a sort of chamber, beneath which the eggs are protected during the earlier part of their development (fig. 91, 2.) The vitellus of the ovum is entirely employed in the construction of the foetus, which latter, at the moment of its escape from the egg, is of an ovoid or sub-spherical shape (fig. 91, 3), completely unprovided with external members, but enabled to swim vivaciously about in the surrounding water by means of the cilia with which its body is profusely covered, giving it exactly the appearance of an infusorial animalcule; indeed, this may be called the first, or infusorial condition of the young Asterias.

Development of Star fish. 1. Echinaster sanguinolentus seen from below.

Fig. 91. Development of Star-fish. 1. Echinaster sanguinolentus seen from below. 2. The same in profile: a, madreporic plate. 3. Ovarian receptacle containing ova in different states of advancement. 4, 5, 6, 7, 6. Ova exhibiting the progressive segmentation of the yelk. 9. Embryo on its first escape from the egg. 10, 11, 12. Further progress of embryo: a a, club-shaped processes; b, central protuberance. 13, 14, 15, 16, show the gradual development of the ambulatory suckers and the assumption of the radiate form.