In the family Heliasteridae the mutual adhesion between the arms has gone on merely to a slight extent, for the interradial septa are still double.
Skeleton.—Most of the schemes of classification have been founded on the skeleton, largely because the greater number of species have only been examined in the dried condition, and little is known of their internal anatomy or habits. There is, however, this justification for this procedure, that the habits and food of the species (with the exception of the Paxillosa) which have been observed in the living condition appear to be very uniform, and that it is with regard to the skeleton that Asteroidea seem to have split into divergent groups through adopting different means of protecting themselves from their foes.
The description of the various elements of the skeleton will be arranged under the following heads:—(a) Main framework; (b) Spines; (c) Pedicellariae; (d) Ambulacral skeleton.
(a) Main Framework.—The type of skeleton which supports the body-wall of Asterias is called reticulate. As already indicated it consists of a series of rods bound together by bundles of connective-tissue fibres so as to form a mesh-work. This is a very common type of aboral skeleton, but in a large number of Starfish a different type occurs, consisting of a series of plates which may fit edge to edge, leaving between them only narrow interstices, as in the Zoroasteridae, or which may be placed obliquely (as in Asterina) so that they imbricate or overlap one another. In a very large number of Asteroidea the supero- and infero-marginal ossicles are represented by squarish plates even when the rest of the skeleton is reticulate; this is the so-called "phanerozonate" structure, the term "cryptozonate" being used when the marginals are rod-like and inconspicuous. In other cases (Ganeriidae) the whole skeleton of the ventral surface is made of tightly fitting plates, whilst the aboral skeleton is either reticulate or made of imbricating plates. Lastly, the skeleton may be represented only by nodules forming the bases of paxillae (see p. [455]), as in the Astropectinidae, or may be entirely absent over wide areas (Brisingidae).
(b) Spines.—The spines vary more than any other part of the skeleton. They may be close set and small, or few and large, and often bear spines of the second order, or spinelets, attached to them. In Asterias and its allies they are comparatively short, blunt tubercles, covered with thick skin. In the Echinasteridae and Asterinidae they are short and blunt, but they are very numerous and thick set. In the Solasteridae they are long, and arranged in bundles diverging from a common base. Such bundles may be termed sheaves, and starting from an arrangement like this, two distinct lines of modification may be traced. Thus (1) the members of a sheaf become connected by a web of skin, so that the sheaf becomes an umbrella, and successive umbrellas may adhere, so that a supra-dorsal tent is formed (a structure characteristic of the Pterasteridae), or (2) the members of a sheaf may become arranged in a circle round a central vertical axis so that a structure like a capstan is produced, which is called a "paxilla" (characteristic of Astropectinidae, Porcellanasteridae, and Archasteridae). The axis,[[453]] as shown by its development, represents the plate which bore the bundle of spines. Again, the skeleton may consist of plates with a close covering of granules (Pentagonasteridae, etc.). Lastly, in Porania spines are absent, the plates being deeply embedded in a thick leathery skin.
Fig. 194.—Views of portions of the aboral surface of different genera of Asteroidea in order to show the main varieties of skeleton. A, Solaster, showing spines arranged in sheaves; B, Pteraster, showing webs forming supra-dorsal membrane supported by diverging spines; C, Astropecten, showing paxillae; D, Nardoa, showing uniform plating of granules. × 8. (After Sladen.)
(c) Pedicellariae.—These are to be looked on as spines of the second order. In Asterina and its allies they are not present, but groups of little spines arranged in twos and threes, each group being attached to a special small plate, are scattered over the aboral surface; and these on irritation approach one another, and represent the rudiment out of which pedicellariae have been developed. The most perfect form, termed "forcipulate," in which there is a basal ossicle, is found in Asteriidae, Brisingidae, Heliasteridae, Pedicellasteridae, Zoroasteridae, Stichasteridae. There are two varieties of forcipulate pedicellariae, the "crossed" and the "straight," which have been described on p. [432]. In all other cases the pedicellariae are devoid of the basal ossicle, and the two or more spinelets forming the jaws are directly attached to one of the main plates of the skeleton.
Fig. 195.—Different forms of pedicellariae (excluding the forcipulate form, for which see Fig. 186). A, pectinate; B, pectinate; C, valvate; D, pincer-shaped; E, alveolate, from the side; F, alveolate, from above. × 10. (After Sladen.)