The Cladophiurae are represented already in the Upper Silurian by Eucladia, in which, however, the arms branch not dichotomously, as they do in modern forms, but monopodially. There is a large single madreporite.
Onychaster, with unbranched arms, which occurs in the Carboniferous, is a representative of the Streptophiurae.
It will therefore be seen that the evolution of Ophiuroidea must have begun in the Lower Silurian epoch. The Streptophiurae are a few slightly modified survivors of the first Ophiuroids. By the time the Devonian period had commenced, the division of the group into Zygophiurae and Cladophiurae had been accomplished.
CHAPTER XVIII
ECHINODERMATA (CONTINUED): ECHINOIDEA = SEA-URCHINS
CLASS III. ECHINOIDEA
The Sea-urchins or Echinoidea (Gr. ἐχῖνος, Hedgehog or Sea-urchin), which constitute the third class of the Eleutherozoa, have derived both their popular and scientific names from the covering of long spines with which they are provided. At first sight but little resemblance is to be discerned between them and the Starfish and Brittle Stars. They are devoid of any outgrowths that could be called arms; their outline is generally either circular or that of an equilateral pentagon, but as their height is almost always smaller than their diameter, they are never quite spherical; sometimes it is so small that the animals have the form of flattened discs.
All doubt as to the relationship of the Echinoidea to the Starfish is at once dispelled in the mind of any one who sees one of the common species alive. The surface is beset with delicate translucent tube-feet, terminated by suckers resembling those of Starfish, although capable of much more extension. The animal throws out these organs, which attach themselves by their suckers to the substratum and so pull the body along, whilst the spines are used to steady it and prevent it from overturning under the unbalanced pull of the tube-feet. When moving quickly the animal walks on its spines, the tube-feet being little used. The tube-feet are distributed over five bands, which run like meridians from one pole of the animal to the other. These bands are termed "radii," and they extend from the mouth, which is situated in the centre of the lower surface, up to the neighbourhood of the aboral pole. The radii must be compared to the ambulacral grooves on the oral surface of the arms of Starfish, and hence in Urchins the aboral surfaces of the arms have, so to speak, been absorbed into the disc, so that the oral surfaces have become bent in the form of a semicircle. The radii are separated from one another by meridional bands called "interradii," which correspond to the interradial angles of the disc of a Starfish and to the sides of its arms. The small area enclosed between the upper terminations of the radii is called the "periproct," and this corresponds to the entire dorsal surface of the Starfish, including that of the arms.
One of the commonest species of British Sea-urchin is Echinus esculentus. In sheltered inlets, such as the Clyde, it is often left exposed by the receding tide, whilst everywhere on the coast in suitable localities it may be obtained by dredging at moderate depths on suitable ground. In the Clyde it is easy to observe the habits of the animal through the clear still water. It is then seen to frequent chiefly rocky ground, and to exhibit a liking for hiding itself in crevices. Often specimens will be seen clinging to the rock by some of their tube-feet, and, as it were, pawing the under surface of the water with the others. In the Clyde it feeds chiefly on the brown fronds of Laminaria, with which the rocks are covered. In more exposed situations, such as Plymouth Sound, it does not occur in shallower water than 18 to 20 fathoms. At this depth it occurs on a rocky ridge; but in 1899, after a south-west gale, all the specimens had disappeared from this ridge, showing at what a depth wave disturbance is felt.
A full-grown specimen is as large as a very large orange; its under surface is flattened, and it tapers somewhat towards the aboral pole. The outline is that of a pentagon with rounded angles. The spines in Echinus esculentus are short in comparison to the diameter of the body, and this is one of the characteristics of the species.