352 On the Echinoidea or Sea-Urchins. [Sess. 



five pieces, called the " compasses," which spring from one end 

 of the wheel-pieces, and at their bifurcated ends carry the 

 muscular ligaments which bind the whole apparatus firmly to 

 the five staples set round the interior orifice of the mouth. 

 The whole forms a most ingenious piece of mechanism, and is 

 popularly known as "Aristotle's lantern." As the teeth wear 

 away at their sharp edges they are renewed from the basal 

 end, which gradually loses its soft spongy structure, and 

 hardens as it nears the summit. A similar mode of growth is 

 found in the teeth of the Eodentia. The echini are believed 

 to be mostly " vegetable " feeders, browsing on marine alg«. 



From the mouth of the sea-urchin a gullet and stomach 

 conduct to a large convoluted intestine, which winds and 

 doubles round the inside of the shell, and ends in an opening 

 on some part of its surface, — in the egg-urchins, at the ab- 

 oral pole. The oral aperture is always on the inferior surface 

 of the test, but is central only in the " regular " group of the 

 Echinoids, to which the egg-urchins belong. The anal aper- 

 ture, also, which is in the centre of the genital disc, opposite 

 the oral aperture, in the egg-urchins, is found in various posi- 

 tions in the " irregular " group, as in the pea-iirchin of the 

 Firth of Forth, where the oral and anal apertures are both on 

 the inferior surface. A so-called blood-vascular system and 

 heart are also present in the sea-urchins, as well as a complex 

 nervous system, already referred to — a perfection of organisa- 

 tion which we would hardly have expected to find in such a 

 lowly creature in the scale of creation. The interior of the 

 shell is always filled with a fluid which must be something 

 more than sea-water, as it is found to be richly corpusculated, 

 and coagulates when exposed to the air, so that it may repre- 

 sent the blood of higher animals. 



The young of the sea-urchins pass through a strange meta- 

 morphosis. As in the case of the crab the larval form was 

 long thought to be a distinct animal, and was known as a 

 " Zoiia," so the embryo of the sea-urchins, as well as of the 

 star-fishes, was at one time placed under a separate group of 

 animals, under the name of a " Pluteus." Joli. Midler was 

 the first to notice these singular larval forms, and the name 

 " Pluteus " was applied by him to the embryo — " from its re- 

 semblance," it has been said, " to a painter's easel with his 



