SEA-URCHINS. 



39 



JAWS OF STONE-URCHIN (nat size). 



rounding the mouth-opening, through which may be seen five pointed teeth. 

 These belong to a very elaborate masticating apparatus, shown in the illustration, 

 and found in all the regular urchins, as also in the Clypeastrida among the 

 irregular urchins. This consists of twenty principal pieces arranged into a five- 

 sided conical mass, compared by Aristotle 

 to a lantern (a). In the centre of the 

 whole are five teeth (6, c), working in 

 bony sockets, or pyramids, connected by 

 muscles with one another, with the interior 

 of the test, and with the arched processes, 

 known as auricles (d), that surround the 

 mouth - opening. There are yet other 

 calcareous pieces connecting the pyramids 

 together, and serving as attachments for yet other muscles. Such a sea-urchin 

 as that described, preserves as much as any echinoderm the five-rayed symmetry 

 of the group ; but in many forms the five-rayed type is not so obvious, for the 

 animal has become elongated along one of the axes, so as to have a superficial two- 

 sided symmetry. This is naturally connected with constant movement in one 

 direction, as though the animal had a head and tail ; and such modification is found 

 among those urchins that live on muddy bottoms, and especially in those from 

 considerable depths. Not only is the test elongated, but the mouth moves forward 



to the front margin, and the vent down- 

 wards to the hinder margin, so as 

 eventually to lie on the under instead 

 of on the upper surface of the test. An 

 earlier stage in this modification is 

 shown in the illustration of the shield - 

 urchin (Echinarachnius), and a fully 

 developed one in the heart -urchin 

 (Brisso>sis), with its long tube -feet 

 extended in the act of walking towards 

 the left. These heart-urchins, as they 

 move along through the sand and mud, 

 scoop it up into their mouths, and pass 

 it through the intestine, extracting on 

 its passage such nutriment as the 

 minute organisms it contains can afford. 

 To enable them to scoop it up in this 

 way, the hinder margin of the mouth 

 is produced forwards in a kind of shovel shape, as is shown in the illustration of 

 a Pourtalesia test from which the spines have been removed. These animals live 

 at very great depths in the sea, and are the urchins most modified in this particular 

 direction. Urchins of the heart-shaped type have short delicate spines, and move 

 almost entirely by their long tube-feet, in the manner described ; but the greater 

 number of the regular urchins progress chiefly by the aid of their spines, which 

 are much stouter, while the tube-feet often have the suckers very imperfectly 



SHIELD-URCHIN, FROM ABOVE (nat. size). 



