Echinodermata in which the calcareous skeleton attains great perfection and bears spines fixed or free, whilst the muscular system is much reduced. The apical system, always present in development, may be obscured by the formation of other plates and is always relatively small. Oral system either absent or represented by five oral plates. The oral surface is ventral in locomotion.
Echinozoa with spherical, heart-shaped or shield-shaped bodies, sometimes much flattened dorso-ventrally. The plates of the calcareous skeleton are well developed and usually immoveably joined edge to edge, and carry moveable spines. The tube feet have terminal discs supported by calcareous plates, but are often much modified. The mouth lies in a peristome, and is usually central but may be displaced in the anterior radius. The anus lies either within the apical system (endocyclic) or outside it in the posterior inter radius (exocyclic).
The outer surface of the body is covered by a ciliated ectoderm and connective tissue layer which contains amoeboid pigment cells and a nerve-plexus. The apical system is always well developed. It consists typically of the usual plates - a dorso-central, surrounded by five basals and five radials. Under-basals and primary single interradials are never present. Among Desmosticha the dorso-central persists as a single plate only in some Sale?iiidae. It is usually replaced by or broken up into a series of irregular plates among which the anus opens. In Clypeastroidea and Petalosticha it is not traceable in the adult at any rate1. The basals are large and are pierced each by one genital pore, or in Palaeo-echinoidea by 3-5 pores. One - the posterior - is generally imperforate in Clypeastroidea and always in Petalosticha, and it appears to be absent (? aborted) in the majority of the latter group. The radials are largest in the old types such as Cidaridae and Saleniidae, smaller in the Petalosticha, and smallest in the Clypeastroidea, the most recent group of Echinoidea. They are each perforated by a pore which transmits the azygos tentacle and the nerve to the eye speck.
The pore is double in many Palaeo-echinoidea. From their relation respectively to the genital apertures and the eye specks the basals and radials are generally termed genitals and oculars. The plates of the ambulacra abut against the radials, of the interambulacra against the basals, and new plates in both these series are added only at the margin of the apical system, between it and the plates already present. And as new plates are added successively, the older plates recede towards the peristome. Consequently they extend in meridional lines from the apex to the margin of that area. One ambulacrum is anterior; one inter-ambulacrum posterior, and easily recognisable by the presence of the anus in it in exocyclic forms, i. e. Clypeastroidea and Petalosticha. In these forms a plane of bilateral symmetry is established through the two meridians named. The ambulacra, one on either side the anterior ambulacrum, constitute with it the trivium: the remaining two, one on either side the posterior interambulacrum, make up the bivium. It has been found that the plates in the ambulacra immediately bordering the peristome show certain constant peculiarities in the trivium and bivium, and it is consequently possible to recognise the corresponding tracts in the endo-cyclic forms, i. e.
Desmosticha, which are characterised by their regular and radial symmetry 1. Bearing this plane of bilateral symmetry in mind, and imagining the Echinoid as lying with the peristome downwards, there are two interambulacra on the left and right sides, and the right anterior basal is somewhat enlarged and porous forming the madreporite. In some Clypeastroidea the madreporite extends into the centre of the apical system and in the majority all the basals and radials are porous and the sutures between them fused. In this case the genital apertures are frequently displaced outside the apical system in the interambulacra. In many Spatangidae the madreporite extends backwards into the posterior interambulacrum, thus dividing the apical system, and in this case the left anterior basal may become porous. The genital pore of the right anterior basal is sometimes lost in this family; so also, but more rarely, that of the left anterior if porous. Consequently as the posterior basal and its pore are absent in all Petalosticha, certain Spatangidae possess only two genital pores (and glands) - viz. those corresponding to the right and left posterior interambulacra. The apical system retains the typical disposition of its plates in Desmosticha and Clypeastroidea. It is then termed compact.
But among Petalosticha, the plates in most Ananchytidae are arranged in two antero-posterior rows touching one another in the middle line; and in the Collyritidae ( = Dysasteridae) the two radials corresponding to the two posterior ambulacra or bivium are separated by a considerable distance from the other plates of the apical system to which they are connected by two irregular rows of supernumerary plates. In the former case the apical system is said to be elongated, in the latter disjointed.
1 It is figured in Echinobrissus scutatus. (Zittel's Palaeontologie, Abth. 1, Palaeozoologie, i. Fig. 388 d. p. 528).
1 If a Spatangus be placed mouth upwards - the reverse of the natural position - and the ambulacrum in the bivium to the left of the observer be numbered I and that to the right V, and the remaining ambulacra II, III, IV, from left to right; and if the two plates in each ambulacrum bordering the peristome be lettered a and b, following the same direction from left to right, then it will be found that the plates I a, II a, III b, IV a, V b are large and are pierced by a double pore, while the plates I b, II b, III a, IV b, V a are small and pierced by one pore. The basal plate of the interradius between II and III is the madreporite. Taking this same plate and interradius as a guide, Loven has shown that if the ambulacra and plates in any other Echinoid be marked in the same manner, the same sequence of large and small plates with more and fewer pores may be noted. Even in the Cidaridae, where the pores are alike on all plates, a difference of size, etc. is noticeable. It may be added that the larger plates in the bivium are placed symmetrically with reference to the odd interambulacrum. See Loven, A. N. 39, 1873, or Etudes sur les Echino'ides, Kongl. Svenska Vetenskap-Akademiens Handlingar. xi.
No. 7. Stockholm, 1874; also Agassiz, Challenger Reports, iii. 1881, for a criticism.
The ambulacra II and III are the two that form the bivium in the Holothurioid, I, IV, V the three that form trivium.