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.
(482). After the lapse of a few days certain appendages begin to make their appearance, sprouting, as it were, from the anterior part of the body, and ultimately appearing as four club-shaped processes (fig. 91,10,11,12,13, a a) surrounding a fifth prominent protuberance, b, whereby the little creature fixes itself to the sides of the incubatory cavity. The body of the little Star-fish now becomes gradually flattened into a minute circular disk, upon one surface of which - hence at once distinguishable as the ventral - the rudiments of tentacula begin to be apparent, under the form of minute globular protuberances, disposed in ten concentric rows (fig. 91,14,15, c c).
(483). If in this condition the little being is detached from the spot where it has fixed itself, it is still able to swim about in the surrounding water by means of its ciliated surface, always keeping the organs of attachment directed forwards; but if left undisturbed, it remains perfectly still and motionless, presenting what M. Sars denominates the crinoid state of development. At this stage, the body of the young Star-fish may still be said to be bilateral; for in all its movements the organs of attachment are directed forwards, and both sides of the body correspond exactly to each other (fig. 91,12.) But by degrees this bilateral condition is converted into the radiated form that characterizes the third, or perfect condition of the Asterias; the body gradually assumes a pentagonal outline, from the angles of which short blunt rays begin to project (fig. 91,16); and the tentacula, now presenting the form of retractile cylinders, and completely furnished with their terminal suckers, become efficient instruments of locomotion. The red spots, regarded by Ehrenberg as the eyes, are visible at the extremities of the nascent rays; the mouth shows itself in the centre of the ventral aspect of the body; and numerous spines make their appearance. Lastly, the apparatus for attachment begins to diminish in size, and soon completely disappears, so that the young Asterias, having attained its perfect form, is ready to enter upon the duties of its station.
(484). According to the observations of Agassiz, the eggs of the Starfish, after they are laid, are taken up by the parent animal and kept between its tubes below the mouth. The Star-fish bends itself around them, surrounds the eggs with its suckers, and moves about with them. When the eggs have been removed to some distance from the animal, it has been observed to go towards them, take them up again, and move off with them, - showing that these creatures, so low in structure and apparently deprived of all instinct, really watch over their young. As the growth of the embryo commences, the external crust of the germ becomes more transparent, consisting of somewhat looser and larger granules, and the internal mass assumes a darker colour, so that two layers become distinct, between which a third is developed. On one side of the germ a protuberance now becomes visible, and the prominent portion separates more and more from the spherical mass, assuming by degrees the form of a peduncle. At this period there is not any organ formed - only changes of substance have taken place; but now little swellings appear in five points on the sides, and the spherical portion of the germ becomes flattened by lateral dilatation.
(485). The minute animal has grown to a more hemispherical shape; and from this time there is an upper and a lower surface to its umbrella-like disk, and a tubular part and a swollen portion to the peduncle. As soon as the peripheric part of the disk begins to spread, five small tubercles may be observed forming underneath; and into these tubercles the peculiar aspect of the middle one extends. Soon other prominent swellings appear, two to each of the former ones, and subsequently two more. While this is going on, calcareous nets are formed by the accumulation of crystals in the cells of the germ. At first there are simply isolated crystals, formed as nuclei in the cells; then several close together will unite and form a little irregular mass; and they will at last combine, so as to constitute a network of solid substance, arranged very regularly, and gradually becoming more and more numerous, marking out more and more distinctly the rays of the embryo Star-fish. The tubercles of the lower surface, growing more prominent and elongated, are finally transformed into the suckers, or ambulacral tubes. With the addition of new calcareous nets, these latter become more numerous, and form, finally, rows of tentacles. Other changes have also taken place.
The cells within the peduncle have undergone alteration: some have become moveable, and a kind of circulation is going on in them. The internal space along each ray has become more transparent; the ambulacral tubes have become hollow; and from that time there seems to be a communication between the external water and the internal structure. What remains of the yelk is more distinctly circumscribed in the centre of the animal, extending as a star-shaped disk into the rays. The radial portion becomes, finally, distinct from the central one; and we have at last an internal cavity, which is the stomach, from which the caecal appendages of the rays, with their liver-like organ, will be developed.
(486). The peduncle is reduced to a mere vesicle; a hole is formed in the centre of the lower surface, constituting the mouth; around this a circular thread becomes visible, answering to the nervous system, from which other threads extend towards the extremity of the rays; and by the time the young Star-fish has attained the size of about a line in diameter, it has thus assumed the form and structure of a perfect animal.
(487). Among the most interesting contributions to our knowledge of this group are the researches of Professor Muller* relative to the embryonic condition of the Ophiuridce, from which it has been ascertained that, during the progress from the egg to the mature condition, the individuals belonging to that family undergo a series of changes that are truly surprising in their character.