55 8 THE ANIMAL KINGDOM. 



rounds the anus. It is produced by resolution of the dorso-central plate 

 in the Desmosticha. In the exocyclic forms it is placed on the dorsal side 

 either above the margin of the test, or on the margin or below it, and its 

 position shifts during growth. 



The test is lined by a layer of connective tissue and the ciliated 

 epithelium of the coelome, which is continued over the surfaces of the 

 contained organs. Its inner surface bears calcareous processes or pillars, 

 except in Petalosticha. In the Scutellidae the dorsal and ventral surfaces 

 of the test are thus closely united. In the Clypeastridae the pillars surround 

 the petala, and one or more pair arise from the interambulacra close to the 

 peristome. Of these the largest are the auriculae. The Desmosticha also 

 possess a circle of auriculae, rising from the interambulacra in the Cida- 

 ridae, extending also on to the ambulacra in ectobranchiate forms in 

 which, as a rule, they form a perfect arch under which pass the radial 

 water-vascular vessels and nerves \ 



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 cal- 

 careous pedicle covered by ciliated epithelium, attached to minute tubercles 

 which form regular bands known as fascioles or semitae. Fascicles sur- 

 rounding 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 some- 

 times surrounded by a circle of similar glands 3 . Pedicellariae are absent 



1 See Martin Duncan, ' Perignathic girdle,' &c. 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 



