THE MUSEUM 
OK 
NATUEAL HISTOEY. 
ZOOLOGY. 
Sub-kingdom IV. — RADIATA (Rayed or Radiated Animals). 
This sub-kingdom of animals contains a great variety 
they breathe by branchite or gills, and the greater 
of creatures, differing from each other in form and 
portion of them are free. Their organs of motion, 
outward appearance. Some are highly organized, 
when thej’’ are present, are movable spines attached 
while others have a very simple structure, and appear 
to the skin, or flexible papillae capable of inflation. 
to form the connecting link between the animal and 
The rayed arrangement of the organs and different 
vegetable kingdoms. Generally speaking, however. 
parts of their body in the greater number, is what has 
they may be distinguished from the three sub-king- 
obtained for the class to which these animals belong 
doms already treated of in this work, by the animals 
the name of Radiata ; and in many of them this is so 
having all the parts of their body and members dis- 
distinct as to give them the shape of the flowers of 
posed in a rayed or radiated form round the digestive 
plants. 
cavity. The organs of digestion consist of a single 
This sub-kingdom is divined into five classes — 
sac, or short alimentary canal. The nervous sj^stem 
1. The Echinoderms or Spiny Skins {Echinodermata). 
is obscure, and in some cannot even be traced. In those 
II. The Acalephs or Sea-nettles (Acalephce). 
in which it exists it is without ganglia, and is composed 
III. The Zoophytes or Polyps {Zoophytu or Polypi). 
of single filaments which are disposed in a circular 
IV. The Infusory and Foraininiferous Animals {Pro- 
form around the orifice of the mouth. They have no 
tozoa.) 
true circulating system, and as they are all aquatic. 
V. The Sponges {Spongia oi Porifera.) 
Class L— ECHINODERMS, or SPINY SKINS (Echinodermata).* 
The Spiny Skins are radiated animals which have 
animals belonging to this class are all marine, and live 
the body covered with a hard coriaceous skin, under 
on animal food. The greater number are free, living 
which or attached to it are frequently found plates of 
unattached, and moving about in the water with their 
solid or calcareous matter, forming a sort of skeleton, 
mouths beneath ; but some are attached, being affixed 
and which in some species is covered with sharp spines 
by their backs, and have their mouths above, so as to 
or prickles, like those of a hedgehog. They possess a 
enable the limbs to bring the food within their reach. 
distinct alimentary canal, distinct organs of circulation. 
The sexes are distinct, and the young are produced 
and a nervous sj'stem forming a ring, generally sur- 
from ova. 
rounding the mouth, and giving off radiate branches. 
Such is a general description of the class Ecbinoder- 
The principal character of the class, however, is, that 
mata ; but in it we find animals of extremely dissimilar 
the animals have, fixed to the skin, a more or less 
appearance associated together. 
considerable number of tentacular-shaped cirrhi, at 
The class has been divided into five orders — 
once serving the purpose of locomotive organs, organs 
1. Crinoidea — Feather-stars. 
of respiration, and organs of touch. In some, as in 
11. Asteroidea — Star-fishes. 
the Echinidse, in addition to the tentacular-formed 
III. Echinoidea — Sea-urchins or Sea-eggs. 
organs of motion and touch, there exist organs, called 
IV. Holothurioidea — Holothurias or Sea- cucumbers. 
pedicellaricc, which serve as organs of prehension. The 
V. Sipunculoidea — Sipunculi. 
As those occupying the centre of this group may bo 
• Derived from the two Greek words ecMnos (6;!:<voe)i ® spine. 
looked upon as the types or models of the radiated 
and derma; skin. 
class, we will begin with them. 
