392 Echinoldea. KADIATA. Echlnid^. 
Order I. — ECHINOIDEA {Sea-eggs, or Sea- 
urchins.) 
The animals belonging to this order (see Plates 1-4, 
Echinodermata) are covered with a round or oval hard 
case, composed of perpendicular bands of square or 
six-sided pieces sunk in the substance of the skin. 
Externally this case is studded all over with numerous 
spines which are affixed on hemispherical tubercles by 
particular muscles, and easily fall off when the animal 
is dead (see fig. annexed). These spines are various 
in shape and size, move in all directions; and — ^judg- 
ing on the principle that Nature does nothing in vain, 
and that even the apparently insignificant peculiarities 
of form which she has given to creatures are designed 
to serve some important end connected with their 
wellbeing — appear to be useful in protecting the ani- 
mals from their enemies, and enabling them also to buiy 
themselves in the sand on the shores when they are left 
by the retiring tide. Half the number of the perj)en- 
dicular bands of which the case is composed, alternating 
with the others, are pierced with minute double pores 
thi ough which are sent out small filaments with dilated 
ends, that enable the animals to anchor themselves to 
marine bodies. These pierced bands, from a fancied 
resemblance to the walks in a garden, are called 
ambulacra; while the alternate ones which have the 
tubercles have been compared to the beds of the gar- 
den, and have in consequence been sometimes called 
ai'em pidvilli (see Plate 1, fig. 4). These animals 
have two separate openings to their digestive canal, 
and in some of the species the vent is placed in 
the centre of the lower surface, whilst in others it is 
situated on the side of the body. The mouth is armed 
with very complicated jaws, and furnished with five 
projecting teeth (see Plate 1, fig. 3 ; Plate 3, figs. 5, 6). 
These jaws were well described by the ancient natural- 
ists, and are known by the name of the “ lanterns of 
Aristotle,” that author having represented them as 
resembling a lantern. The animals belonging to this 
order are carnivorous, living chiefly upon small mol- 
lusks and Crustacea. The recent species are numerous, 
and are found in most parts of the world ; while the 
fossil remains of their shells or external cases are 
abundant in many of the formations, but especially in 
the chalk. 
The numerous species of which the order is com- 
posed, may be divided into six families : — 1, Echinidce; 
2, Cidaridce ; 3, Scutellidce ; 4, Galeritidce ; 5, Echi- 
nolampasidce ; and 6, Spatangidce. 
Family I. — ECHINIDiE {True Sea-eggs), 
This family is perhaps the most numerous in species, 
contains some of the best known and most commonly 
distributed, and may be taken as the most typical of 
all. The shell or case is generally rounded and com- 
posed of twenty perpendicular bands of square or six- 
sided pieces. The tubercles upon which the spines 
are placed are simple, rounded, im- 
perforate ])rominences. The spines 
vary a good deal in shape. Gener- 
ally, as in Echinus, they are equal- 
sized, conical and striated. In 
others, as in Colohocentrus, they are 
large and club-shaped ; while in a 
third set, as in Helerocentrus, they 
are very short and truncated, form- 
ing in the mass a smooth surface 
— somewhat resembling a tesse- 
lated pavement. The ambulacra 
are constantly of the number of five 
pairs (see Plate 2, fig. 3), and are 
complete, extending from the upper 
to the lower surface. In addition 
to the spines on the tubercles and 
the filaments which are sent out 
from the ambulacral pores, and 
which serve as organs of pro- 
gression, the Echini possess other 
movable organs scattered over the 
suiface of the bod)^ and known by 
the name of Pedicellarice. These 
oigans though small, are very curi- 
ous, and perform a very important 
part in the economy of these ani- 
mals. They are prehensile, and are used by the animal 
to seize hold of objects, which can thus be passed 
from one to another, and possibly may even be con- 
veyed from the dorsal surface to the mouth. They 
are very numerous, crowded irregularly on most parts 
of the skin, but especially round the oral aperture. 
They are of various forms, but in general they may be 
described as having a thick head, cleft into three 
divisions, and set on a long, slender, flexible stalk. 
The whole body and head are invested with a glutin- 
ous flesh, in which are imbedded minute red glands, 
that are common to the integument of the whole class. 
Fig. 234. 
Ecliinus. 
