The oral system of plates is represented in Palaeostoma mirabilis, one of the Spatangidae, by five large interradial plates surrounding the mouth, but it is doubtful if this system exists in other Urchins.
The five ambulacral and interambulacral areae make up the corona or test. The typical structure of the test is as follows: - Each of the ten areae consists of two rows of pentagonal plates, less numerous but larger in the interambulacra. The pentagon is so disposed that an angle is turned to the central line of union of the two rows in each area and a flat side to the line of union between adjoining areae. The plates are firmly united by their edges and the test is therefore resistent: a suture indicates the line of union. Each plate is perforated by a double pore. Hence the term pore-plate. The two pores give exit to two processes from the ampulla within the test which unite and form a tube foot. The important variations from the type structure are the following. Among Palaeo-echinoidea, Bothriocidaris has but a single row of interambulacral plates, whilst the number is increased in the remaining groups, as is the case also in some genera in the ambulacra. The central rows in each area are in these instances hexagonal. Among living Urchins Astropyga (Diadema-tidae) and some species of Phormosoma (Echinothuridae) have an increased number of interambulacral plates.
There is an overlapping of the plates in Palaeo-echinidae, in Astropyga, and Echinothuridae, a group in which it is well marked, and the test as a rule very flexible. An overlap is slightly indicated also in Spatangidae, a group in which the two rows of plates in the posterior interambulacrum not only are somewhat separate, but can be approximated by muscles. In the same family the pore is single on the majority of the plates except those at the apical and oral extremities. The middle region of the expanded plates in the petaloid portion of the ambulacra of Clypeastroidea may be marked by rows of fine pores in addition to the two large pores connected by a furrow (yoked pores) at the outer edge. And the remaining plates of the ambulacra may possess similar rows which may extend on to the interambulacral plates forming pore-areae. In the family Scutellidae these fine pores are arranged on the actinal surface in lines - -pore-fasciae - sometimes branched. The pore plates in Cidaridae, Clypeastroidea, and Petalosticha retain their regular succession. In the two last named the dorsal sections of the ambulacra gradually dilate from the apex and then contract more or less completely near the margin of the test forming petala, arranged in a rosette.
In Petalosticha however the anterior ambulacrum is often unlike the rest, not much expanded, and its pores small or obliterated. The remaining sections of the ambulacra have plates so much expanded laterally in Clypeastroidea that the peri-stomial ends of the interambulacra are often constricted or excluded from the peristomial margin. In the Cassidididae the peristomial ends of the ambulacra dilate into petala or phyllodes, forming a figure known as Jioscella. In the Ectobranchiate Desmosticha on the contrary the pore-plates at a little distance from the apex lose their typical arrangement. Certain plates (half plates) remain small in size and fail to reach the central line of the ambulacrum: others increase in size, and there is a fusion of three or more primary plates to form secondary plates. Near the peristome the secondary plates themselves may fuse. As to the interambulacral plates, they become much expanded near the peristome of Spatangidae, especially in the posterior meridian, where the single plate ending the series is expanded laterally, forming the labrum. It is preceded by two broad elongated plates, the sterna, and these by the episterna. The whole structure constitutes a raised plastron.
The interambulacral plates in the typical Clypeastroidea and Petalosticha are irregular in shape 1.
The test as a rule retains a regular outline. In the Scutellidae, however, notches often correspond to the ambulacra and to the posterior inter-ambulacrum at the margin of the flattened disc. They may be converted into foramina during growth, and are sometimes very numerous in the region of the bivium.
The peristome is membranous. It is covered by plates continuous with the ambulacra and interambulacra respectively, and set free from the corona in the Cidaridae. In ectobranchiate Desmosticha there are five pairs of perforated radial buccal plates close to the margin of the mouth, and the rest of the membrane is either soft or irregularly plated. The same radial perforated plates are present in Clypeastroidea with the addition of a single interradial between each pair. The peristome becomes more or less excentric during growth in Petalosticha and in the most typical family, Spaiangidae, transversely elongate as is the mouth itself. It has imperforate plates. A membranous area or periproct covered by irregular plates surrounds 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.
1 For the 'tag,' see Martin Duncan, A. N. H. (5), xvi. 1885.
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 nerves1