4i6 



ANIMAL RESPIRATION 



organs has increased and they have become more or less specialized 

 gills (fig. 544), branched in some cases so as to give a larger 

 surface. 



The structures which correspond to tube-feet in Brittle-Stars 

 (Ophiuroids) project from the sides of the arms, and are of no 

 use for locomotion. They probably assist in breathing to some 

 small extent, but their chief use is to act as sense org-ans. 



Most of the Sea-Cucumbers (Holothurians) are provided with 

 tube-feet which help the other breathing organs present, but they 



fig- 545. — Dissection of Sea-Cucumber (diagrammatic) 



The water-vascular ring (w.v.r.J, bearing stone-canals (sT.c.) and Polian vesicles (p. v.), sends branches to 

 the tentacles (tent.), and gives rise to five longitudinal radial vessels (r.w.v. ) which bear tube-feet not 

 shown in figure) ; e.t., r.t., the two respiratory trees opening Into cloaca ; cuv., Cuvierian organ, from 

 which sticky defensive threads are ejected. 



are not so important in this respect as the circlet of tentacles at 

 the front end of the body, which also contains branches of the 

 water-vascular system (fig. 545). 



ZOOPHYTES (Ccelenterata) 



The Hydroid Zoophytes, Jelly-Fish, Sea-Anemones, Corals, 

 and Comb-Jellies which make up this large subdivision of the 

 animal kingdom breathe by the general surface of the body, and 

 the mouth is often surrounded by one or more circlets of tentacles, 

 which, while their main use would appear to be to secure food 

 (see p. 156), also largely increase the area over which breathing 

 can take place. This is particularly necessary in those cases 

 where there is a firm skeleton by which more or less of the 

 surface of the body is prevented from taking part in respiration. 



