7i6 



ECHINODERMATA. 



RADIATA. 



ECHINODERMATA. 



We 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 investiga- 

 tion that their rea.l position is discovered. 



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

 dermata. 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 

 hedge-hog. In these animals the radiate form is very plainly shown, some of them as- 

 suming 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 structureless gelatine. 



The mode of walking, or rather creeping, which is practised by these beings, is very 

 interesting, and may be 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 sud- 

 denly 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 being thrust out to a considerable distance, and sometimes withdrawn almost wholly 

 within the shell. 



These are the " ambulacras, " or walking apparatus, and are among the most ex- 

 traordinary means of progression in the animal kingdom. Each of these innumerable 

 organs acts as a sucker, its soft head being applied to any hard substance, and adher- 

 ing 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 con- 

 tinually 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. 



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



ON the upper portion of the illustration is seen a specimen of the turritella-shell, from 

 which protrudes a long, slender, worm-like being. This is the HERMIT SIPUNCULUS, a 

 creature which is remarkable for the fact that it resides in the empty shells of molluscs, 

 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 link between these two great divisions, for in its person radiism sets and an- 

 nulism begins. The end of the body, which is concealed within the shell, is capable 

 of being enlarged into a bulb-like shape, which enables the creature to maintain a firm 

 hold of its shelly retreat, and the other extremity is furnished with an external proboscis, 

 at the end of which is a small circlet of tentacles. 



Several species of this genus are eatable and held in great estimation by the Chinese, 

 who catch them in a very ingenious manner. The EDIBLE SIPUNCULUS lives in holes in 

 the sand, and always keeps the mouth of its burrow open. The Chinese fishermen arm 

 themselves with a bundle of slender wooden rods, tapered to a point at one end, and 





