THE ECHINUS, OR SEA-URCHIN. 
369 
with spicula. These suckers, being very extensible, can be 
protruded and withdrawn ; and by their aid locomotion is indeed 
mainly effected. 
There is a third and most singular group of organs scattered 
over the whole surface of the buccal membrane and corona. 
They are of minute size, so that to observe them requires a 
handglass, and they were formerly taken to be parasites. They 
are termed pedicellarice , and are complex organs, being even 
each furnished with a delicate internal skeleton. Each consists 
of a long slender stalk, ending in three short limbs (or jaws), 
which diverge from the distal end of the stalk. These jaws 
open and shut with a snapping action, while the stalk sways 
about. 
The utility of these organs is not yet fully determined, but 
they have been observed to remove particles of excrement, 
passing them along, as it were, from hand to hand. They 
may, however, also pass along nutritious particles to the mouth, 
and they may serve to detach foreign bodies, ova, &c., which, 
if allowed to remain, would grow up parasitically and injure 
the echinus. They singularly remind us of the bird’s-head 
processes of certain polyzoa or bryozoa (polyp-like animals, with 
ciliated tentacles). 
On the inside of the corona, at the lower end of each ambu- 
lacrum, is a solid calcareous arch, termed an auricula , while 
within the space bounded by these five auriculae is that singu- 
larly complex masticating organ termed u Aristotle’s lantern ,” 
made up of a score of distinct parts. When, in the unmutilated 
animal, the mouth is looked at, the apices of five pointed teeth 
are seen to protrude more or less. 
Each of these teeth is externally very like a cutting tooth 
of a rat or squirrel, though much simpler in minute structure. 
It is constantly reproduced from a soft root at the upper end 
(where it is enclosed in a secreting bag) as it is worn away at 
the apex. 
Each tooth is enclosed in a composite* vertical plate, folded 
on itself and interambulacral in position. These are the alveoli . 
The adjacent upper edges of the alveoli are held together by 
strong horizontal pieces, which pass from one alveolus to the 
next, are oblong in shape, and are called the rotulce. Each 
rotula is opposite an auricular arch. 
The last set of pieces are the five radii , which are long 
slender parts fixed to the apices of the rotulae like springs, 
extending upwards and outwards, and bifurcating at their 
ends. 
All these complex parts act together, and separate and ap- 
* 11 Composite ” because it consists, primitively, of four parts. 
VOL. IX. — NO. XXXVII. B B 
