RADIATES. 155 



All these classes commence in the Lower Silurian ; and some of 

 their subdivisions are therefore here mentioned. 



1. Echinoderms. — The subdivisions of Echinoderms are as 

 follow : — 



1. Holothurioids. — Having the exterior soft, and throughout ex- 

 tensile or contractile, and the body stout, subvermiform in shape. 

 The group is not known among fossils. It includes the Biche de 

 mar, or Sea-slug. 



2. Echinoids. — Having a thin and firm hollow shell, covered ex- 

 ternally with spines (fig. 153) ; form, flattened spheroidal to disk- 

 shape; the mouth below, at, or near the centre, as the Echinus, 

 fig. 153. 



3. Asterioids. — Having the exterior stiffened with calcareous 

 pieces, but still flexible ; form, star-shaped and polygonal ; mouth 

 below, at centre ; animal free, except in the young state. Ex., the 

 Star-fish, fig. 154. 



4. Crinoids. — Crinoids are related to the Asterioids and Echi- 

 noids, but have, with few exceptions, a permanent stem or 

 pedicel, as figs. 155, 156, 158. They are thus like the young Aste- 

 rioids. The stem is attached to the back, and they stand with 

 the mouth upward. Fig. 155 represents the Crinoid closed, like 

 a closed bud; when opened, it would appear like an opened 

 flower, and each ray would be seen to be delicately fringed, as 

 in fig. 158. 



A. Echinoids. — Fig. 153 represents an echinus partly uncovered 

 of its spines, showing the shell beneath, and 157 another, wholly 

 uncovered. The shell consists of polygonal pieces in twenty ver- 

 tical series arranged in ten pairs. Five of these ten pairs are per- 

 forated with minute holes, and are called the ambulacral series [a 

 in fig. 153 represents one pair) ; and the other five alternating with 

 these are called the inter-ambulacral (b). The inter-ambulacral 

 areas have the surface covered with tubercles, and the tubercles 

 bear the spines, which are all movable by means of muscles. The 

 ambulacral have few smaller tubercles and spines, or none; but 

 over each pore (or rather each pair of pores) the animal extends 

 out a slender fleshy tentacle or feeler, which has sometimes a 

 sucker-like termination and is used for clinging or for loco- 

 motion. The ambulacral areas are thus distinguished from the 

 others by being generally much narrower, by having smaller 

 spines or none, and by having a multitude of these tentacles 

 or feelers, — the use of which is partly for aiding the animal in 

 its motions, partly for seizing food, and partly to supply vesicles 

 in the interior with water for the purposes of respiration. In 



