THE MUSEUM 



NATURAL HISTORY 



ZOOLOGY. 



Sub-kingdom IV. — RADIATA (Rayed or Radiated Animals). 



This sub-kingdom of animals contains a great variety 

 of creatures, diiiering from each other in form and 

 outward appearance. Some are higlily organized, 

 wliile otliers liave a very simple structure, and ajipear 

 to form the connecting link between the animal and 

 vegetable kingdoms. Generally speaking, however, 

 they may be distinguished from the three sub-king- 

 doms already treated of in this work, by tlie animals 

 having all the parts of their body and members dis- 

 posed in a rayed or radiated form round the digestive 

 cavity. The organs of digestion consist of a single 

 sac, or short alimentary canal. The nervous system 

 is obscure, and iu some cannot even be traced. In those 

 ill which it exists it is without ganglia, and is composed 

 of single filaments which are disposed in a circular 

 form around the oritice of the mouth. They have no 

 true circulating system, and as they are all aquatic, 



they breathe by branchiai or gills, and the greater 

 portion of them are free. Their organs of motion, 

 when they are present, are movable spines attached 

 to the. skin, or flexible papilla; capable of inflation. 

 The rayed arrangement of the organs and difl'erent 

 parts of their body in the greater number, is what has 

 obtained for the class to which these animals belong 

 the name of Radiata ; and in many of them this is so 

 distinct as to give them tie shape of the flowers of 

 plants. 



This sub-kingdom is diviocd into five classes — 



I. The Echinoderms or Spiny Skins {Echinodermatn). 



II. The Acalephs or Sea-nettles {Acalcphce). 



III. The Zoophytes or Polyps {ZoLrphyta or Polyjii]. 



IV. The Infusory and Foraminifcrous Animals {Pro- 

 tozoa.) 



V. The Sponges {Spongia oi Pori/cra.) 



Class I.— ECHINODERMS, or SPINY SKINS (Echinodermata).* 



The Spiny Skins are radiated animals which have 

 the body covered with a hard coriaceous skin, under 

 which or attached to it are frequently found plates of 

 solid or calcareous matter, forming a sort of skeleton, 

 and which in some species is covered with sharp spines 

 or prickles, like those of a hedgehog. They possess a 

 distinct alimentary canal, distinct organs of circulation, 

 and a nervous system forming a ring, generally sur- 

 rounding the mouth, and giving ofl' radiate branches. 

 The principal character of the class, however, is, that 

 the animals have, fixed to the skin, a more or less 

 considerable number of tentacular-shaped cirrhi, at 

 once serving tlie purpose of locomotive organs, organs 

 of respiration, and organs of touch. In some, as in 

 the Echinidae, in addition to the tentacular-formed 

 organs of motion and touch, there exist organs, called 

 pedicellaricc, which serve as organs of prehension. The 



* Derived from the two Greek words cch'mos (>x'>o(), a spine, 

 and ijcrma (Jii^fjia.), skin. 



animals belonging to this class arc all marine, and live 

 on animal food. The greater number arc free, living 

 unattached, and moving about in the water with their 

 mouths beneath; but some are attached, being aflixcd 

 by tlieir backs, and have their mouths above, so as to 

 enable the limbs to bring the food within their reach. 

 The sexes are distinct, and the young are produced 

 from ova. 



Such is a general description of the class Echinodor- 

 mata ; but in it we find animals of extremely dissimilar 

 appearance associated together. 



The class has been divided into five orders — 



I. Crinoidea — Feather-stars. 



II. Asteroidea — Star-fishes. 



III. Echinoidea — Sea-urchins or Sea-eggs. 



IV. Ilolothurioidea — Holothurias or Sea encumbers. 



V. Sipunculoidea — Sipunculi. 



As those occupying the centre of this group may be 

 looked upon as the types or models of the radiated 

 class, we will begin with them. 



