412 



ANIMAL RESPIRATION 



united by their edges, and frequently bearing spines. (2) The 

 possession of a set of tubes collectively known as the "water- 

 vascular system ", this name having been given because the 

 system is connected with the exterior, for the purpose, it would 



Fig. 542. — Mouth-area of a Sea-Urchin [Echinus escidenUis)^ enlarged 



?«., Mouth with five teeth; }nb.^ membrane surrounding moutii: o.g.^ an oral gill; ^., specialized tube-foot; 



i/-t ordinary tube-feet. 



appear, of taking in sea-water. The most important parts of the 

 water-vascular system are a ring round the gullet, one or more 

 canals by which this is connected with the outside of the body, 

 and five branching tubes which it gives off to the "arms", rays, 

 or equivalent parts of which the body largely consists. 



INFLUENCE OF THE SKELETON ON THE DEVELOPMENT 

 OF BREATHING ORGANS 



It is quite evident that the possession of a firm external 

 skeleton must more or less prevent breathing by means of the 



