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.
Fig. 93. 1. Ophiura in a still more advanced stage of development, showing the larva portion (Pluteus) in great part obliterated: first appearance of the mouth and tentacles. 2. The larva has entirely disappeared, and the feet and spines of the Ophiurus begin to develope themselves. 3. Shows the mode of growth of one of the rays: the terminal or primitive segment is easily recognizable, to which the following segments succeed in the order of their formation. (After Midler).
(496). In order to complete the history of the Asteridae, we have yet to mention the nervous apparatus wherewith they are furnished. This consists of a simple circular cord that runs around the mouth of the animal; from this ring, three delicate filaments are given off opposite to each raj, one of which, according to Tiedemann, runs along the centre of the amhulacral groove upon the under surface of the body, and gives off minute twigs to the locomotive suckers placed on each side of its course; the other two filaments pass into the visceral eavity, and are probably distributed to the internal organs. There are no ganglia developed on any part of this nervous apparatus; or if, as some writers assert, ganglionic enlargements are visible at the points whence the radiating nerves are given off, they are so extremely minute as not in any degree to merit the appellation of nervous centres.
(497). Such an arrangement can only be looked upon as serving to associate the movements performed by the various parts of the animal; for no portion of these simple nervous threads can be regarded as being peculiarly the seat of sensation or perception. Nor is this inference merely deducible from an inspection of the anatomical character of the nerves; it is based upon actual experiment. We have frequently, when examining these animals in a living state (that is when, with their feet fully developed, they were crawling upon the sides of the vessels in which they were confined), cut off with scissors successive portions of the dorsal covering of the body, so as to expose the visceral cavity; but, so far from the rest of the animal appearing to be conscious of the mutilation, not the slightest evidence of suffering was visible: the suckers placed immediately beneath the injured part were invariably retracted; but all the rest, even in the same ray, still continued their action, as though perfectly devoid of participation in any suffering caused by the injury inflicted. Such apathy would indeed seem to be a necessary consequence resulting from the deficiency of any central seat of perception whereunto sensations could be communicated.
Nevertheless Ehrenberg insists upon the existence of eyes in some species of Star-fish, attributing the function of visual organs to certain minute red spots, visible at the extremity of each ray, behind each of which he describes the end of the long nerve that runs along the ambulacral groove as expanding into a minute bulb. We must confess that the proofs adduced in support of such a view of the nature of the spots appear to us to be anytliing but satisfactory. The general sense of touch in the Asteridae is extremely delicate, serving not only to enable them to seize and secure prey, but even to recognize its presence at some little distance, and thus direct these animals to their food. Any person who has been in the habit of fishing with a line in the shallow bays frequented by star-fishes, and observed how frequently a bait is taken and devoured by them, will be disposed to admit this; yet, to what are we to attribute this power of perceiving external objects? It would seem most probably due to some modification of the general sensibility of the body, allowing of the perception of impressions, in some degree allied to the sense of smell in higher animals, and related in character to the kind of sensation whereby we have already seen the Actiniae and other polyps are able to appreciate the presence of light, although absolutely deprived of visual organs.
(498). The Echini, however they may appear to differ in outward form from the Asteridae, will be found to present so many points of resemblance in their general structure, that the detailed account we have given above of the organization of the last-mentioned family will throw considerable light upon the still more elaborately constructed animals that now present themselves to our notice.
(499). The Echinidae, as we have already observed, differ from the star-shaped Echinodermata in the nature of the integument that encloses their visceral cavity, as well as in the more or less circular or spherical form of their bodies; so that the locomotive apparatus with which they are furnished is necessarily modified in its character and arrangement.
(500). The shell of an Echinus (fig. 94, l) is composed of innumerable pieces accurately joined together, so as to form a globular box enclosing the internal parts of the animal, but perforated at each extremity of its axis by two large openings, one of which represents the mouth, and the other the anus.
(501). The calcareous plates entering into the composition of this extraordinary shell may be divided into two distinct sets, differing materially in shape, as well as in the uses to which they are subservient. The larger pieces are recognizable in the figure by hemispherical tubercles of considerable size attached to their external surface, adapted, as we shall afterwards see, to articulate with the moveable locomotive spines. Each of these larger plates has somewhat of a pentagonal form, - those that are situated in the neighbourhood of the mouth and anal aperture being considerably the smallest, and every succeeding plate becoming progressively larger as they approximate the central portion of the shell: the entire series of pieces in each row resembles in figure the shape of the space included between two of the lines marking the degrees of longitude on a terrestrial globe - broad at the equator, but gradually narrowing as it approaches the poles, - an arrangement, of course, rendered necessary by the spherical form of the creature.