GENERAL CHARACTERS OF RADIATA. 35 



shelly matter, so as to form a protective habitation, within which the animal 

 can withdraw its body, but which is by no means to be regarded as a part of 

 it, and does not exhibit any definite type of form. In the Radiata, all the parts 

 are arranged in a circular manner, the mouth being in the centre ; some of 

 them are protected by firmly-jointed external skeletons, like those of the Arti- 

 culata ; whilst others deposit calcareous matter in the centre of their soft fleshy 

 structures, as if sketching out the internal skeleton of the Vertebrata. The 

 skeletons of most of the Invertebrata differ, however, from those of Vertebrate 

 animals, in this important character, that they are not permeated by vessels, 

 and are formed only by a superficial deposition. Hence they are termed extra- 

 vascular : and it is an obvious result of an arrangement of this kind, that parts 

 once formed are never changed, except by the ordinary processes of decay, and 

 that they can only be extended by addition to their exterior ; whilst in Verte- 

 brata, the bones are subject to alterations of any kind, whether of removal or 

 addition, throughout their entire substance. It is not correct to regard them, 

 however, as destitute of vitality ; since they consist, in all instances, of a regu- 

 larly-organized tissue, in which the mineral matter, where such exists, is 

 deposited ; and in several cases they are traversed by tubes, which seem to 

 convey a fluid destined for their nutrition, if not actual blood. Structures of 

 this kind are on the same footing with the dentine and enamel of the teeth of 

 Vertebrata, ( 633, 634) ; to which they sometimes bear a very strong resem- 

 blance. A more detailed account of the general structure of these sub-king- 

 doms will now be given, beginning with the lowest. 



General characters of Radiata. 



18. The RADIATA possess many points of affinity with the Vegetable king- 

 dom ; and of these, the circular arrangement of their parts is one of the most 

 evident. Many species of Sea-Anemone, for instance, present an appearance 

 so much resembling that of various composite blossoms, as to have been com- 

 monly termed Animal-flowers, a designation to which they further seem 

 entitled, from the small amount of sensibility they manifest, and the evident 

 influence of light upon their opening and closing. But it is in the tendency 

 to the production of compound fabrics, each containing a number of indivi- 

 duals, which have the power of existing independently, but which are to a 

 certain degree connected with one another, that we recognize the greatest 

 affinity in structure between this group and the Vegetable kingdom. Every 

 tree is made up of a large number of buds, which are composed of leaves ar- 

 ranged round a common axis ; each bud has the power of preserving its own 

 life, and of reproducing the original structure, when removed from the parent 

 stem, if placed in circumstances favourable to its growth ; and yet all are con- 

 nected in the growing tree, by a system of vessels, which forms a communica- 

 tion between them. This is precisely the nature of those structures, which 

 are formed by the animals of the class that may be regarded as the most charac- 

 teristic of the group. Every mass of coral is the skeleton of a compound 

 animal, consisting of a number of polypes, connected together by a soft flesh, 

 in which vessels are channeled out ; these polypes are capable of existing 

 separately, since each one, when removed from the rest, can in time produce a 

 massive compound fabric, like that of its parent ; but they all contribute to the 

 maintenance of the composite structure, so long as they are in connection with 

 it. In some instances, the skeleton is stony, and is formed by the deposition 

 of calcareous matter, either in the centre of each fleshy column, so as to form 

 a solid stem, or on its exterior, so as to form a tube. In other cases, it is horny; 

 and then it may be a flexible axis, or a delicate tube. Both the stony and 

 horny corals frequently possess the form of plants or trees : and as their skele- 



