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.
(451). In the generality of Star-fishes, the arrangement, and indeed the entire character of the calcareous plates, differs materially in different parts of the body; and even in the same species considerable modifications are observable. In the coriaceous integument forming the dorsal parietes of the animal, the pieces in many cases seem rather to be represented by calcareous granules disseminated through the interior of the skin; in other cases they are arranged in lines anastomosing with each other in all directions, so as to represent, when the skin is dried, a rude network of solid particles, upon the exterior of which the various cutaneous appendages already noticed are sustained.
(452). It is, however, upon the ventral aspect of the Asterias that the skeleton assumes its most perfect development; the floor of every ray is made up of a continuous series of detached pieces, or vertebrae, as they are generally called, fitted to each other and united by a strong ligamentous substance, so as to form a succession of joints, upon which the flexibility of the ray depends. The pieces around the mouth constitute a strong circular framework enclosing the oral aperture, from which, as from the centre, the rest of the skeleton radiates. The joints forming the floor of the ray succeed to this; these are partially represented in fig. 90, where, the soft parts having been removed from the ray marked b, their general arrangement is displayed.
(453). The vertebrae, as they are called, when thus exposed are found to be individually composed of several pieces, and each is articulated by oblique facets to those which precede and follow it, - a kind of union that admits of considerable motion, and provides for the flexibility of the ray, so as to render it capable of executing movements requisite for the purpose of progression, or of seizing prey. The connexion of the vertebrae is effected in such a manner, that between each pair of calcareous plates minute orifices are left, which in the entire state of the ray are seen to be arranged in a quadruple series; these holes give passage to the locomotive suckers, and from this circumstance have been named the ambulacra! holes, while the furrows, seen upon the ventral surface, into which they open are designated the ambulacral grooves.
(454). The suckers, which at the will of the animal are protruded through the ambulacral apertures, forming the principal agents whereby locomotion is effected, next require our notice. In the annexed figure (fig. 87) they are seen fully extended, projecting for some distance beyond the margins of the ambulacral grooves that occupy the middle of each ray, every one of them being furnished at its extremity with a sucking disk, adapted to take firm hold upon any smooth surface. The mechanism whereby these suckers, or feet, as they are usually called, are extended from the body and again retracted, is very simple. That portion of each foot which is external to the shell is a muscular tube, closed at one extremity, namely that whereunto the sucker is appended; whilst by the opposite it communicates, through the corresponding ambulacral hole, with a globular contractile vesicle situated within the body of the animal. Both the tubular foot, and the vesicle appended to it, are endowed with a power of independent action; so that, if the vesicle contracts, the fluid within it is forced into the external tubular portion of the organ, which thus becomes distended and rendered erect; but if, on the other hand, the muscular tube shrinks in turn, the contained fluid is forced back again into the internal vesicle, and the whole foot collapses.
The arrangement referred to will be easily intelligible on reference to the annexed rough diagram, which represents a longitudinal section of one of the rays of the Asterias depicted above. The internal vesicles (fig. 88, l, h) occupy the floor of each segment of the body; and when viewed from above (fig. 90, d), the entire series resembles strings of transparent beads placed above the rows of ambu-lacral apertures, through which they communicate with the tubular feet (fig. 88, 1, g.) In fig. 88, 2, three of these organs are represented in different states of extension, and their whole structure is developed. The foot (d) is shown protruded to its full extent; the vesicle, much contracted, has forced the fluid which it contained into the external tube (i), whereby it is rendered tense and prominent. The muscular coats that invest the exterior of the protruded portion are likewise depicted; the internal layer (k), immediately in contact with the membranous canal continued from the vesicle, is made up of longitudinal bands passing from the root of the organ towards the sucker at its extremity, while the outer layer (I) consists of circular fibres, - an arrangement evidently adequate to the performance of all required movements.
Fig. 87. Astenas.
Fig. 88. 1. Diagrammatic section of a Star-fish: a, mouth; b, stomach; c, intestiniform caeca; d, dorsal surface; e, ambulacral plates; f, ovarium; g, tubular feet; h, internal vesicles. 2. Diagram representing the suckers in different states of extension: a, ambulacral plates; b, c, d, internal vesicles; e,f, g, vascular system; i, tubular foot, laid open to exhibit k, I, its muscular coats.
(455). The other portions of this diagram represent the feet in different stages of protrusion: in fig. 88, 2, c, the vesicle being partially contracted, the tubular portion is seen in a medium state of distention; and at b, the sucker is shown in a still more retracted state, the contained fluid having been completely expelled from the muscular tube and driven back into the vesicle, which is distended to the utmost.