ECHINOIDEA. 559 



from the peristome of Cidaridae, and are not found in Clypeastroidea nor in 

 Petalosticha with the exception of Echinoneus 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 Cassidulideae (? 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 modi- 

 fied 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. 



The circumoral nerve ring lies beneath the peristomial membrane 

 and the radial nerves internally to the ambulacral plates which are there- 

 fore 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 com- 

 pound 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 Desmo- 

 sticha. 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. 



