RADIATA, OR ECHINODfiRMATA. 



E now arrive at a vast and comprehensive division of living beings, which have no 

 joints whatever, and no limbs, and are called Radiata, because all their parts 

 radiate from a common centre. The structure is very evident in some of these 

 beings, but in others the formation is so exceedingly obscure, that it is only by 

 anatomical investigation that their real position is discovered. 



The highest forms in this division have been gathered together in the class 

 Echinodermata. This word signifies Urchin-skinned, and is given to the animals 

 comprising it because their skins are more or less furnished with spines, resembling those of 

 the hedgehog. In these animals the radiate form is very plainly shown, some of them assum- 

 ing a perfectly star-like shape, of which the common star-fishes of our coasts are familiar 

 examples. In some of the Radiates, such as the sea-urchin, the whole body is encrusted with 

 a chalky coat, while in others it is as soft and easily torn as if it were composed of mere struc- 

 tureless gelatine. 



The 7node of walking, or rather creeping, which is practised by these beings, is very inter- 

 esting, and maybe easily seen by watching the proceedings of a common star-fish when placed 

 in a vessel of sea-water. At first it will be quite still, and lie as if dead, but by degrees the 

 , tips of the arms will be seen to curve slightly, and then the creature slides forward without 

 any perceptible means of locomotion. If, however, it be suddenly taken from the water and 

 reversed, the mystery is at once solved, and the walking apparatus is seen to consist of a vast 

 number of tiny tentacles, each with a little round transparent head, and all moving slowly but 

 continually from side to side, sometimes being thrust out to a considerable distance, and some- 

 times withdrawn almost wholly within the shell. 



These are the "ambulacra',"' or walking apparatus, and are among the most extraordinary 

 means of progression in the animal kingdom. Each of these innumerable organs act as a 

 sucker, its soft head being applied to any hard substance, and adhering thereto with tolerable 

 firmness, until the pressure is relaxed and the sucker released. The suckers continually move 

 forward, seize upon the ground, draw the body gently along, and then search for a new hold. 

 As there are nearly two thousand suckers continually at work, some being protruded, others 

 relaxed, and others still feeling for a holding-place, the progress of the creature is very regular 

 and gliding, and hardly seems to be produced by voluntary motion. 



The Echinodenns of our North American coast are not conspicuous for beauty, and, con- 

 sequently, are not well known popularly. At this day it is probable that very few persons 

 that visit the sea-shore and pick up one of the common sea-urchins that are seen there, would 

 have the slightest conception what it is — indeed, most people would question its kingdom — 

 whether vegetable or animal. Tn the absence of information, no wonder ; but is it not an 

 undesirable state of things, that the nature of the most common objects of our sea-shore should 

 be so completely unfamiliar I 



We will now proceed to our examples of these curious beings. 



The Hermit Sipuxculus {Sipunculus hernhardus) is a long, slender, worm-like being. 

 It is a creature which is remarkable for the fact that it resides in the empty shells of mollusks, 

 after the same fashion as is observed by the hermit crabs. 



If taken out of the shell, the Sipunculus resembles a. worm so closely, that it might easily 

 be mistaken for an annelid ; and, indeed, according to one of our best zoologists, it forms a 



