RADIATED TYPE OF STRUCTURE. 



67 



is not confined to their exterior, but extends to the internal 

 organs also ; which are so constructed, that each is but a repeti- 

 tion of the rest. Thus, in every arm of the Star-fish, we have 

 not only the same number of the little plates of which the 

 skeleton is composed, and the same arrangement of the small 

 tubular feet which are put out between these, but we have also 

 in each a nervous ganglion and trunk, a prolongation of the 

 stomach, and a set of blood-vessels, all of which are precisely 

 similar in the several rays. 



46. This repetition of similar parts around a common centre 

 strongly reminds us of the arrangement of the leaves in a leaf- 

 bud (VEGET. PHYSIOL. 303) 

 and of the sepals, petals, &c. 

 in a flower (VEGET. PHYSIOL. 

 463). In the Polypes, which 

 constitute the largest and 

 most important of the groups 

 exhibiting this plan of struc- 

 ture, a number of individuals, 

 each capable (like a leaf-bud) 

 of living by itself, are arranged 

 on one common plant-like 

 structure (Fig. 27) ; and this 

 extends itself by budding, in 

 the manner of a tree or shrub. 

 It is not only in their exter- 

 nal aspect, and in their ten- 

 dency to form compound 

 structures, that the animals 

 exhibiting the radial symme- 

 try bear a resemblance to 

 Plants; for it shows itself 

 also in the predominance of 

 their vegetative over their 

 animal life, that is, in the 



large proportion which their organs and actions of nutrition bear 

 to those of sensation and locomotion. The greater part of the 



F 2 



FIG 27~ SERTULARIA. 



