Fig. 94. - Cidaris papillata. (After Gosse.)
Sides the spines, which are sometimes of a very great length, the test hears curious little appendages, called "pediiariae" (fig. 93, F), and originally supposed to be parasitic.
Each of these consists of a stem, bearing two or three, sometimes four, blades or claws, which snap together and close upon foreign objects like the beak of a bird. Their action appears to be independent of the will of the animal, and their true function is not known; but they may be regarded as peculiarly modified spines. One function performed by the pedicellariae, in some species at any rate, is the removal of excrementitious particles of food. Such particles, on being ejected from the vent, are seized by the pedicellariae, passed on from one to another, and ultimately entirely got rid of.
In almost all recant Urchins, the test also carries, as shown by Loven, curious stalked appendages, with button-like heads covered with cilia. These so-called "sphaeridia" are supposed to be organs of sense - probably of taste.
Locomotion in the Echinoidea is effected by means of a singular system of contractile and retractile tubes, which constitute the "ambulacral tubes," or "tube-feet," and are connected with the "ambulacral system" of aquiferous canals (fig. 95). From the perforated "madreporiform tubercle" on the largest of the genital plates, there proceeds a membranous canal, known as the "stone" or "sand canal" (s), whereby water is conveyed from the exterior to a circular tube (r) surrounding the oesophagus, and constituting the centre of the water - vascular or ambulacral system. The function of the madreporiform tubercle (m) appears to be that of permitting the ingress of water from the exterior, but of excluding any solid particles which might be injurious; and as its area is much larger than that of the stone-canal, it admits sea-water not only to the ambulacral vessels, but also to the body-cavity. It should be added, however, that the admission of water to the body-cavity through the madreporic tubercle is denied by Perrier. The "circular canal" (r) surrounding the gullet is situated between the nervous and blood-vascular rings, and gives off five branches - the "radiating canals" - which proceed radially along the "ambulacral areas" in the interior of the shell (a a). In this course they give off numerous short lateral tubes - the "tube-feet" - which pass through the "am-bulacral pores" to gain the exterior of the test, and terminate in suctorial discs. la-sides the radiating ambulacral canals, there are connected with the circular canal certain vesicles of unknown functions (p p), known as the "Polian vesicles" (ampullae Polianae). Five Polian vesicles are generally present; but some forms are wholly without these organs. The ambulacral tubes, or tube-feet, can be protruded at the will of the animal through thepores which perforate the ambula-cral areas, and can be again retracted. By means of these locomotion is effected, the tnbe-feet being capable of protrusion to a length greater than mat of the longest spines of the body. The mechanism by which the tnbe-feet are protruded and retracted is as follows: Each tube-foot, shortly after its origin, gives rise to a secondary lateral branch, which terminates m a vesicle. These resides or "ampullae" (v) are provided with circular muscular fibres, by the contraction of which their contained fluid is forced into the tube-feet, which are thus protruded. Retraction of the ambulacral tubes is effected by proper muscular fibres of their own, which expel again the fluid which has been forced into them by the vesicles. The walls of the stone-canal are strengthened by calcareous deposits; and the terminations of the tube-feet contain in many forms a calcareous rosette, often with a calcareous ring below it, whilst the walls of the tube-feet are furnished with calcareous spicules.
The total area over which the tube-feet can be protruded depends upon the extent to which the "ambulacral" or "poriferous" zones of the test are developed. In the typical or "Regular" Sea-urchins, the ambulacral areas are "perfect," and extend from pole to pole; whereas in the so-called "Irregular" Urchins (such as the Heart-urchins and Cakeurchins), they are "interrupted," being restricted to the summit of the test, and usually being broad and petaloid (fig. 93, D).
Fig. 95. - Diagram of the ambulacral system of Echinus. m Madreporiform tubercle; s Stone-canal; r Central oesophageal ring; p p Polian wades; a a Radiating ambu-lacral vessels. Only the bases of four of the radiating vessels are shown; and a few of the tnbe-feet (t) with their secondary vesicles or "ampulae" (v), are shown on one side of one of the radiating canals.
As regards the digestive system, the mouth is typically situated in the centre of the base; but it may be excentric; and in one singular living form (Leskia) it is protected by valvular calcareous plates. Some forms have the mouth toothless, but others possess a complicated masticating apparatus. In Echinus this consists of five long, calcareous, rod-like teeth, which perforate five triangular pyramids, the whole forming a singular structure, known as "Aristotle's Lantern" (fig. 96, C).