To the plates of the corona are attached various structures - spines, clavulae, pedicellariae, and sphaeridia. The spines are generally restricted to the interambulacra in Palaeo-echinidae. They are attached to tubercles which vary in size in accordance with the size of the spines themselves, and are perforate or imperforate centrally. The larger tubercles are surrounded by a smooth area - the scrobicula - and this by a ring of smaller scrobicular tubercles which bear correspondingly small spines. The spines vary much in shape and in the structure of the calcareous stem 2. When first formed they are similar both in shape and structure in all groups alike. The Petalosticha retain the primitive condition, as do some Clypeastroidea and a few Desmosticha. But in the oldest genera and families the spines increase in size and vary much in structure. The attachment takes place by a socket, and a capsular muscle. Clavulae, found only in Spatangideae, are minute spines with expanded head and a calcareous pedicle covered by ciliated epithelium, attached to minute tubercles which form regular bands known as fascioles or semitae. Fascioles surrounding the petaloid ambulacra or making a ring below the anus are historically the earliest to appear.
The pedicellariae are modified spines with a calcareous, sometimes jointed, pedicle, bearing three, rarely four valves. The form of the valves is very variable and serves to distinguish different kinds of pedicellariae. They are opened and shut by special muscles. On the inner face of each valve of the pedicellariae globiferae there is a gland, and at its base a touch-organ. The pedicle is also sometimes surrounded by a circle of similar glands 3. Pedicellariae are absent from the peristome of Cidaridae, and are not found in Clypeastroidea nor in Petalosticha with the exception of Echinonens and the Spatangidae. Sphaeridia are absent only in Cidaridae, and occur on the peristome and ambulacra. They are freely exposed in Desmosticha and most Spatangideae, lodged in some of the latter in depressions of the test which in others, in Clypeastroidea and Cassididideae (? all) are converted into closed cavities. They increase in number with the age of the individual, are renewed like the spines, and like them are moveably connected to the test. As they are at once set in motion by any chemical change in the sea-water they are probably olfactory and gustatory in function.
Structurally they are modified spines, spherical or oval in shape, and composed of calcareous laminae perforated by irregular channels. These channels are filled with branched nervous cells connected to nerves which enter the globule at its base. The nervous cells end in club-shaped projections lying between the epithelial cells covering the exterior in a single layer. The epithelial cells are ciliated in patches.
1 See Martin Duncan, 'Perignathic girdle,' etc. J. L. S. xix. 1885.
2 In the soft Urchins (Echinothuridae) the larger spines are swollen at the apex, and inflict a painful and poisoned wound. For the structure of the poison apparatus, see C. B. and P. F. Sarasin, Z. A. ix. 1886, p. 81.
3 Hamann has described under the name of 'Globiferi' what appears to be pedicellariae
The circumoral nerve ring lies beneath the peristomial membrane and the radial nerves internally to the ambulacral plates which are therefore superambulacral in position. There is a plexus within the test which governs the coordinate movements of the spines. Branches pass out with the tube feet and at their bases, are connected to an external plexus with ganglion cells, from which filaments pass up the stalks of the pedicellariae and innervate the valve-muscles. At the tips of the feet they end in the terminal disc in a plexus connected apparently with sense-cells. The structure of the eyes lodged on the ocular plates does not seem to be accurately known. A Diadema has recently been found in which compound cellular eyes are distributed on the genital plates, interambulacra, ambulacra, and round the bases of the spines. The animal is sensitive to changes of light and shade. There is a circumoral blood-vascular and water-vascular ring, which lie at the base of Aristotle's lantern in Desmosticha. The structural details of the blood-vascular system are much disputed.
The presence of a plexiform organ ( = ovoid gland) connected with the oral ring is generally admitted, and there is little doubt that radial trunks exist not only in Spatangus but in other Urchins. The plexiform organ extends to the madreporite and has been stated to open globiferae, in which the valves have become obsolete, leaving only the glands. They are found in many Urchins (A. N. H. (5), xvii. 1886, p. 386). Martin Duncan has discovered in some of the bodies in question atrophied valves (op. cit. (5), xviii. p. 67). Hamann also finds that the inner aspect of the valves in all kinds of pedicellariae is covered, in addition to ciliated cells, with sense-cells bearing setae and connected to nerve-fibrils. In the ped. globiferae the sense-cells are collected into one or more elevations, sometimes also into an apical elevation as well (A. N. H. (5), xvii. p. 469). Cf. Martin Duncan's remarks, op. cit. xviii. p. 68. The muscles of the valves of pedicellariae are transversely striated (Hamann, op. cit. p. 388; Beddard, ibid. p. 428). For lit. of pedicellariae, see p. 194. externally through it (?). There are two intestinal vessels, one - the ventral vessel - coursing along the inner side of the tract, the.other - the dorsal - on the outer.
They only extend a certain distance along the intestine and are connected by capillaries. The ventral vessel opens into the oral ring. The water-vascular ring is connected to radial trunks which pass on the outer side of the masticatory organs, whilst the corresponding blood-vessels are said to pass on the inner side upon the walls of the pharynx in Desmosticha. The ring has five Polian vesicles in Desmosticha, each placed in an interradius, and many vesicular appendages in Clypeas-troidea. The Polian vesicles appear to contain blood-vascular plexuses. The stone-canal is membranous in Echinus, calcareous in Cidaris, and extends, as usual, to the madreporite. It has been stated recently that it becomes continuous with the plexiform organ in Spatangus. The tube-feet are connected to ampullae within the test, and are typically furnished with a sucking disc supported by a calcareous rosette and by pieces in the walls of the feet themselves. The feet belonging to the buccal plates of the peristome end not in a disc but in two or more processes. The dorsally placed feet in Desmosticha are often branched and respiratory. The fine pores of Clypeastroidea emit minute feet of the ordinary structure, the yoked pores of the petala flat pinnate respiratory feet.