SEA-URCHINS. 161 



water-feet in the ambulacral grooves. Zoroaster sigsbeei and Z. ackleyi are two 

 species dredged by the 'Blake' in or near the Caribbean Seas, in 1878-79. The last 

 attains a diameter of about nine inches across the arms. Though the suckers are 

 ranged in four rows at the base of the arms, there are but two rows at the extremity. 

 It greatly resembles an Ophidiaster in general appearance. 



The small size of the sucking-discs of the water-feet, and the general aspect of the 

 animals, suggest that the genus should be placed with the Astropectinidae, near to 

 Luidia. 



Caidaster is a singular star-fish, furnished with a peduncle in the centre of the-disc, 

 suggestive of the centro-dorsal tubercle of a comatula. It was taken by the French 

 exploring expedition. 



Class III. — ECHINOIDEA. 



The Sea-Urchins are echinoderms without arms and without a stalk. They are 

 protected externally by a calcareous test, composed of plates known as the coronal 

 plates. These plates form ten distinct areas, five of which are perforated with pores 

 for the exit of the suckers, and are known as the ambulacral areas, while the five 

 intermediate areas bear no suckers, and are known as interambulacral. The variations 

 in the form and size of these plates, and of the areas they compose, give rise to the 

 varying shapes of the different genera. The mouth is always situated upon the lower 

 or actinal aspect, which is applied in progression to the surface upon which the animal 

 moves ; but the position of the anus varies in different families. The surface of the 

 plates, except where the pores are situated, is covered with spines, which occur upon 

 every species of echinoid, but vary greatly in their number, structure, and size, so 

 that they form one of the best characters by which species can be distinguished. 

 There seems at first sight to be little resemblance between the huge bat-like spines 

 of Heterocentrotus, the sharp, hollow, brittle spines of the Diadematidse, the solid- 

 fluted spines of the Echinidae, and the slender, delicate spines, usually short, but 

 sometimes long and silky, of the spatangoids or irregular sea-urchins, yet A. Agassiz 

 assures us that in their early stages the young spines of Echinids are much alike. 

 They are polygonal, made up of rectangular meshes placed in regular stories one above 

 the other. There is no difference in the typical structure of the spine of the young of 

 Cidaris, Echinus, Strongylocentrotus, Arbacia, EcM- 

 nocyamus, or Schizaster. Some recent genera, espe- 

 cially the spatangoids, retain a type approaching 

 that of all young echini, while among many older 

 genera, as in Cidaris and other regular sea-urchins, 

 complicated types occur. 



The coronal plates are more or less pentagonal, 

 and are usually firmly united at their edges. Twenty 

 principal longitudinal series, two in each ambulacra], 

 and two in each interambulacral area, make up the 

 mass of the test, and a series or rosette of ten single 

 plates form a ring round the aboral or apical margin, fig. mo. — EcMnaracimius parma ,• a, am- 

 The apical extremities of the ambulacra abut upon '"'^^"'''' Mnter-ambuiaorai areas. 

 the five smaller of these plates, each of which is perforated, supports the eye- 

 spot, and is called the ocular plate. The apical ends of the inter-ambulacra cor- 

 VOL. I. — 11 ' 



