THE SEA-UECHINS. 269 



The arms of the Ophiuricls are rather appendages to the body (Fig. 11) than actual portions of it, 

 as is the case in the Asterids (Fig. 1). The greater part of each arm is formed by a central bony 

 axis, which is composed of successive joints, and fills up almost the whole of the internal cavity of the 

 arm. Each of the quadrate axial ossicles (Fig. 12, A 6 ; Fig. 13, ao) consists of two lateral halves, 

 which are united in its middle line, and represent the smaller and less-developed 

 ambulacral ossicles in the arm of a Starfish (Fig. 9, ao). The successive 

 ossicles are connected by pairs of strong muscular bundles, and articulate by 

 tenon and mortice joints upon their terminal surfaces. 



Corresponding to each ossicle of the internal skeleton are four superficial 

 plates, viz., the "lower arm-plate" (Fig. 12, s 2 ; Fig. 13, 1), the upper "arrn- 

 plate" (Fig. 13, u), and two "side arm-plates" (Fig. 13, s). These plates are 

 often more or less covered with spines, as is shown in the specimen figured 

 (Fig. 11). On either side of the under arm-plate, between it and the side 

 arm-plates, are openings by which the tube-feet reach the exterior. Each 

 opening is protected by a little scale or scales, which may be upon the side 

 arm-plate, or upon the lower arm-plate, or upon both. 



The tube-feet have less to do with locomotion than their fellows in the OPHIURID ARM (slightly 

 Urchins and Starfishes, as they have no terminal suckers, but they are very erd/romSiadeiO. 



* * 00, ambulacra! ossicle; ?t, upper 



sensitive to touch. Their chief function is probably respiratory, while locomotion T w^rlirm-pJat^^T'radiai 

 is effected by means of the worm-like arms, which are capable of a very con- u"radia^water\'essei?a!ara- 

 siderable amount of lateral movement, though they cannot be bent to any great 



extent. The Ophiurids are much more active than the Asterids, and of them the Brittle-stars are more 

 so than the Sand-stars, seldom remaining quiet for a moment, but keeping their arms in a state of 

 continual twisting movement. They also have a singular power of breaking their arms into fragments, 

 which are often flung away to some little distance from the disc, new ones growing out from it 

 again after a longer or shorter interval, for the power of reparation which these animals possess 

 is very considerable. 



Most of the Ophiuroidea have simple and undivided arms (Ophiurida, Fig. 11); but in the 

 members of the order Astrophytida* the arms fork ten or twelve times, and the numerous branches 

 into which they divide interlace with one another, so as to form a sort of trellis- work all round the 

 disc. These creatures are variously known by the names of Basket-fish, Medusa-head Starfish, and 

 Argus. 



The habitat of the Sand-stars may be gathered from their name, while the Brittle-stars are to be 

 found both on a sandy bottom and in the rock-pools on the shore. Many of them are very abundant 

 in the neighbourhood of oyster-beds and scallop-banks, and are largely preyed upon by the cod and 

 other fish, while their own stomachs are full of minute foraminiferal shells. 



(3) Echinoidea. The members of this class are variously known as Sea-eggs, Sea-hedgehogs, or 

 Sea-urchins. The last name, used as it often is without the prefix, is merely a corruption of " Oursin," 

 the French word for hedgehog. This appellation is not bestowed without reason, the body of any 

 common Echiiiid being more or less globular and covered with spines. These spines are jointed on to 

 knobs or tubercles, which are borne by the closely-fitting limestone plates of the test or shell (Fig. 

 14, A, B). The tubercles do not, however, cover the whole surface of the test indiscriminately, but 

 they are chiefly disposed in five broad zones, which extend from one pole to the other. Alternating 

 with these are five narrower zones, which bear smaller and fewer tubercles, and are pierced with small 

 holes arranged in regular rows. Through these holes the Urchin extends its tentacles or tube-feet, 

 which are provided with terminal suckers, like those of the Starfishes, and are largely used in 

 locomotion, especially when the creature is climbing a steep slope. On level surfaces, however, the 

 spines are also brought into play, the animal advancing by a sort of tilting motion. 



Scattered among the spines are pedicellarise, resembling those of the Starfishes, except that they 

 have three prongs instead of two (Fig. 14, c, D). They are said to be used in climbing for laying hold 

 of fronds of seaweed, and so enabling the Urchin to steady itself until it can make use of its sucking 

 feet. They are also employed as scavengers, those round the anal opening laying hold of the ejected 



* Greek, aster, star ; phuton, plant ; eidos, form. 



