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.
* For a detailed account of the fossil Encrinites, the reader is referred to 'A Natural History of the Crinoidea, or Lily-shaped Animals,' by J. S. Miller. 4to, Bristol, 1821.
Setting out from this point to trace the gradual development of organization in the Echinodermata, we shall observe a progressive concentration of their entire structure. The central part, or visceral cavity, so small in the Comatula when compared to the complicated rays derived from it, enlarges in its proportional dimensions as the viscera contained within it become more perfect in their arrangement, whilst, on the other hand, the radiating or polyp form, so visible in Encrimis and Comatula, becomes obliterated by degrees, until, at length, almost all vestiges of it are lost, or but obscurely recognizable.
In the Gorgonoce-phalus (fig. 82), the proportionate size of the rays, when compared with that of the central disk, still preponderates very considerably, although even here some concentration is manifest. The secondary articulated filaments appended to the rays of Comatula are no longer recognizable, their place being supplied by the continual division and subdivision of the rays themselves: the same end, however, is obtained in both cases; for the numerous jointed and flexible rays of Gorgonocephalus still form so many legs, enabling the creature to drag itself along the bottom of the sea, or to entwine itself among the submarine plants, as well as supplying the office of tentacula in securing food.
Fig. 81. Comatula.
Fig. 82. Gorgonocephalus.
(437). Continuing our progress towards more perfect forms of these remarkable animals, we at length arrive at genera in which the rays become divested of all elongated appendages, either in the shape of articulated lateral filaments or dichotomous ramifications. In Ophiura, for instance (fig. 83), the rays are long and simple, resembling the tails of so many serpents - a circumstance from whence the name of the family is derived ; nevertheless, on each side of every ray we still trace moveable lateral spines, which, although but mere rudiments of what we have seen in Comatula, may yet assist in locomotion, or perhaps may contribute to retain the prey more firmly when seized by the arms. The rays themselves are composed of many pieces curiously imbricated and joined together by ligaments, so that they are, from their length and tenuity, extremely flexible in all directions, and serve not only for legs, adapted to crawl upon the ground, but are occasionally serviceable as fins - able to support the animal in the water for a short distance by a kind of undulatory movement. The body, or central disk, is beautifully constructed, being made up of innumerable pieces accurately fitted together. The mouth occupies the centre of the ventral surface, and is surrounded by radiating furrows, in which are seen minute apertures that give passage to a set of remarkable prehensile organs, to be described hereafter: these are calculated to act as suckers, and so disposed as either to fix the body of the animal, or to retain food during the process of deglutition.
Fig. 83. Ophiura.
(438). Leaving the Ophiurae, we are led through a long series of almost imperceptible gradations to the Star-fishes (Asterias) (fig. 87); in these, from the increased size of the body, the rays are united at their origin, and become so much dilated as to contain prolongations of the viscera lodged in their interior - an arrangement not met with in Ophiurae and other slender- rayed Asteridae. The dilatation of the central part proceeds, and in the same proportion the rays become obliterated; so that by degrees, the asteroid shape becomes totally lost by the progressive filling up of the interspaces between the rays, and we arrive ultimately at completely pentagonal forms, the sides of the pentagon being perfectly straight lines.
(439). It is extremely interesting to observe the changes which occur in the nature of the locomotive organs during these diversifications of external figure. We have seen that, in the lower Echinodermata, possessing long and flexible rays, such organs were fully adequate to perform all movements needful for progression; but as the mobility of these parts is diminished by their gradual curtailment and the filling up of the spaces between them, some compensating contrivance becomes indispensably necessary; and accordingly we find an apparatus gradually developed, well calculated to meet the exigences of the case. In Ophi-ura we have already mentioned the existence of protrusible suckers around the opening of the mouth, well adapted, from their position, to take firm hold of food seized by the animal; and it is by increasing the number of such organs that ample compensation is made for the loss of motion in the rays themselves. On examining the lower surface of an Asterias, even in those forms which most approximate a right-lined pentagon in their marginal contour, the number of rays will still be found to be distinctly indicated by as many furrows radiating from the mouth, and indicating the centre of each division of the body.
These "ambulacral furrows," as they are termed, exhibit, when examined in a dried specimen, innumerable orifices arranged in parallel rows, through each of which, when alive, the animal could protrude a prehensile sucker, capable of being securely attached to any smooth surface (fig. 87).
No verbal description can at all do justice to this wonderful mechanism, even leaving out of the question the means by which each individual sucker is wielded (of this we shall speak hereafter); but let any of our readers, when opportunity offers, pick up from the beach one of these animals, the common Star-fish of our coast, which, as it lies upon the sand, left by the retiring waves, appears so incapable of movement, so utterly helpless and inanimate; let him place it in a large glass jar filled with its native element, and watch the admirable spectacle which it then presents. Slowly its tapering rays expand to their full stretch; hundreds of feet are gradually protruded through the ambulacral apertures, and each, apparently possessed of independent action, fixes itself to the sides of the vessel as the animal begins its march. The numerous suckers are soon all employed, fixing and detaching themselves alternately, some remaining firmly adherent, while others change their position; and thus, by an equable gliding movement, the Star-fish climbs the sides of the glass in which it is confined, or scales the perpendicular surface of the submarine rock.