CHAPTER XXXV 



ANIMAL RESPIRATION— LOWER INVERTEBRATES WHICH 

 BREATHE IN WATER 



SEGMENTED WORMS (Annelids) 



The most interesting members of this group, so far as the 

 present section is concerned, are to be found among the Bristle- 

 Worms (Chsetopoda), and after first considering these it will be 

 necessary to add a little regarding Leeches (Discophora). 



BRISTLE-WORMS (Ch^topoda) 



The marine Bristle- Worms (Polychsetes) include both actively 

 carnivorous forms which have no fixed abode, and also tube- 

 dwellers, although there is no sharp boundary between the two 

 sub-groups. The skin is in all cases sufficiently thin to be of 

 use in breathing, and the surface offered for this purpose is in- 

 creased by the presence of numerous pairs 

 of hollow foot-stumps, the organs by which 

 burrowino- crawlino- and sometimes swim- 

 ming are effected. There may, however, be 

 special respiratory organs on the upper side 

 of the body, and these not infrequently take 

 the form of branching gills. In certain flat- 

 tened forms (Polynoids) there is a double 

 series of thin scales (elytra) on the back, and 

 these would appear to answer the same 

 purpose (fig. 538). Such scales may be visible 

 externally, but in the Sea-Mouse [Aphro- 

 dite) they are enclosed in a sort of gill-chamber, which is roofed 

 over by a felt-work of minute bristles cemented together. 



The Lugworm {Arenicold), which lives in a burrow in the 

 sand, is a sort of transitional case between those marine bristle- 



Fig. 538.— Scale-Worm (Poly- 

 noc}~ Front end seen from above, 

 to show breathing-scales. 



