ia mno 
ZOOLOGY. 43 
(interambulacral plates). The arms are pierced with holes, 
hence their name of ambulacra; through these holes or 
ambulacra are protruded the sucker-like bodies just men- 
tioned. There are ten ambulacral plates, arranged in pairs, 
and between these ten interambulacral plates, also in pairs; 
so, starting from right to left, we have two ambulacral plates 
united, then two interambulacral plates united, and so on 
around the shell. The plates are composed of still smaller 
pieces, these minute plates being formed through the 
secreting power of the skin, which dips down between the 
different plates. The shell of an Echinus, with its innumer- 
able pieces, plates, spines, and suckers, is therefore quite a 
complex organism. If we turn to the interior of the animal, 
we find the intestine loosely attached, but possessing in its 
jaws a most complicated apparatus, the so-called Lantern 
. of Aristotle. This lantern-shaped apparatus is composed 
of five triangular pieces, united at their bases. Crowning 
the apex of each triangle is seen a tooth, the sides of the 
triangle being furnished with fine saw-like teeth. The five 
triangles are kept firm by clamps, and movable through 
delicate muscles, the whole forming a most efficient, though 
delicate, arrangement. The nervous system is a simple 
ring surrounding the mouth, with radiating threads; the 
organs of reproduction are arranged in a direction corre- 
sponding to that of the arms. The structure of the Echinus 
is essentially radiate. Suppose, however, that an Echinus 
be drawn out until its length exceeds its breadth, and the 
mouth be encircled by a wreath of tentacles, we would 
have then a sea-cucumber, or Holothuria. (Fig.45.) The 
Echinodermata agree in the structure of their water-vascu- 
lar locomotor system, in the peculiar lining or hardening 
of the skin which incloses their bodies, and in many 
other respects. They may be regarded, therefore, from 
their structure and manner of development, as a distinct 
division of the animal kingdom. The origin of the Echino- 
