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.
* Ueber die Larven und Metamorphose der Ophiuren und Seeigel. Berlin Trans. 1846.
(488). The young Ophiura on leaving the egg presents itself under a most grotesque form, in which condition it has been long known to naturalists, and described under the name of Pluteus, or Easel Animal-ode, from its resemblance to a painter's easel.
(489). The Pluteus paradoxus (fig. 92) is exceedingly minute, being not more than 2/5ths of a line in length. When highly magnified, its body is seen to be somewhat of a conical shape, terminating above in a point, but dividing inferiorly into eight long processes or appendages of various dimensions, to which it owes its peculiar figure (fig. 92, l, a, b, c, d.) Each of these processes is supported by an internal calcareous framework derived from the interior of the body (fig. 92, 1,f), which, branching out in different directions, forms a basis whereon the soft parts are spread out. The whole animal is perfectly transparent, its substance resembling dull glass, the apex of the body and the extremities of the arms or processes being slightly tinged with orange.
Fig. 92. 1. Pluteus paradoxus. A A, lateral arms; B B, inferior ditto; C C, anterior ditto; D D, posterior ditto; a, mouth; a', oesophagus; b, stomach; c, granular bodies, the nature of which is uncertain; d, cseciform appendages, which make their appearance around the oesophagus and stomach, and which are the first indications of the development of the Star-fish; e, ciliated bands; f, calcareous framework of the skeleton; g, zone of cilia surrounding the apex of the body; x, nervous system. 2. Further development of the caeciform appendages, d; they begin to exhibit the appearance of the body or central disk of an Ophiura. (After Muller).
(490). These singularly-formed larvaz - for such they are - are found abundantly during the months of August and September, crowding the surface of the sea in rich profusion, swimming freely about by the aid of rows of cilia (e), with which their arms and the apex of their bodies (g) are plentifully furnished. They possess, moreover, a distinct nervous system, consisting of two little ganglia (x) situated just beneath the oral aperture, from whence delicate nervous threads may be traced in different directions.
(491). The first appearance that presents itself, indicating the commencement of metamorphosis, is the development of a number of caecal appendages around the stomach and oesophagus of the Pluteus (fig. 92, 1, d), which soon increase so much in number that they form a series of rows surrounding the stomachal cavity. At first these rows of caeca do not extend beyond the body of the Pluteus, remaining, as it were, concealed beneath its disk; but soon acquiring greater development, they make their appearance externally, and begin to assume some regularity of arrangement (fig. 92, 2), in which the rudimentary form of the star-fish begins to be perceptible, and the points whence the arms are to proceed become apparent.
(492). In carrying out this part of the proceeding, it will be observed that the original arms or processes of the Pluteus (fig. 92,1, a, b, c, d) have had no share. The Pluteus, in fact, stands just in the same relation to the young Ophiura as the frame does to a piece of embroidery; neither has the structure of its arms anything in common with that of the rays of the future star-fish, which lies, as it were, protected beneath their shelter. As soon as the caecal appendages have arrived at this state of development and assumed so much regularity of arrangement, calcareous earth begins to be deposited in an arborescent form, which accumulates rapidly until a kind of trellis-work is formed, spreading over the entire surface of the young Echinoderm. As the caeciform appendages thus become arranged into a regular figure, the place where the mouth of the Pluteus was becomes distorted and, as it were, forcibly pushed upwards, until it remains no longer visible, its place being occupied by the central mouth of the newly-formed starfish (fig. 93, 2).
(493). In the condition which it has now attained, the young starfish is still much smaller than the rest of the Pluteus; but from this point, as its growth continues, the body and processes of the latter assume more and more the appearance of being only appendages to the newly-developed animal, until by degrees they entirely disappear, the only part of the Pluteus remaining as a part of the young Ophiura being the stomach.
(494). Before, however, the arms of the Pluteus have entirely disappeared, the feet, or retractile suckers, have begun to show themselves, arranged in a circle around the circumference of the shield (fig. 93,1,2), so that it is able to creep freely about in the sea.
(495). Shortly before the disappearance of the last remnants of the Pluteus, the arms or rays of the Ophiura are already visible, projecting prominently from the margin of the shield (fig. 93, 1, 2), but consisting as yet only of the outer or terminal joint of the future ray; the moveable spines likewise begin to show themselves, and the characters of the future Echinoderm begin to be recognizable (fig. 93, l.) Ultimately new segments begin to be added to the rays, making their appearance between the primitive segment and the margin of the disk, the original segment retaining its size and figure unaltered, while the succeeding ones differ in their shape, assuming a polygonal form, which varies in different species. The places where all new segments are formed are in the shield itself, at points situated upon the ventral aspect, between the inter-radial spaces; and each successive segment produced, being at the base of the ray, is of course larger than all that preceded it (fig. 93, 3, 4).