436 GENERAL CHARACTERS OF RADIATA. 



resemblance is manifested, also, in the regular disposition of these 

 parts around a common centre, which may be termed circular 

 symmetry. The tendency to this kind of symmetry exists through- 

 out the Vegetable Kingdom; being most obvious in the arrangement 

 of the parts of the flower (VEGET. PHYSIOL. 463). That the 

 Radiata should preserve the mode of development, which is so 

 remarkably characteristic of the Vegetable Kingdom, is not sur- 

 prising, when we reflect upon the very small proportion which 

 their animal functions bear to those of organic life. None of 

 them possess any high degree of sensibility ; and whilst many 

 of them are fixed like plants during a part or the whole of their 

 existence, none possess any very active powers of locomotion. 



988. Although the radiated form may be observed in certain 

 members of every class which has an undoubted claim to admis- 

 sion into this group, it by no means follows that it should exist 

 in each individual of those classes. Like every other natural 

 character, it is subject to modifications. The species which pre- 

 sent it in the most remarkable degree may be regarded as the 

 typical forms of their respective groups ; whilst others, in which 

 it is less evident or altogether absent, serve as connecting links 

 with those divisions of the Animal Kingdom which are formed 

 upon a different plan. Thus, among the POLYPTFERA, the Sea- 

 Anemone^ and many of the associated animals resembling it, 

 have a most regular arrangement of similar parts, both externally 

 and internally, around a common centre. On the other hand, 

 there is an important group of Polypes, which, with the same 

 circular arrangement of the tentacula or arms round the mouth, 

 presents, in its more complex digestive apparatus, an entire 

 departure from that regularity; and through this group we are led, 

 by almost imperceptible gradations, to the MOLLUSC A. In like 

 manner we have, in the Star-fish^ a perfectly symmetrical dispo- 

 sition of all the organs of the body. The stomach, situated in 

 the centre, sends a prolongation into every arm ; the absorbent 

 vessels which arise from its walls are united in a central ring, 

 and are distributed on a uniform plan in each ray ; the nervous 

 system has a similar central ring, and sends equal branches in 

 every direction ; the locomotive organs are the same in each 



