ANNELID WORMS 239 



yellow, violet, green, or other tint may be due entirely 

 to pigments in or under the skin. Red may be due to 

 haemoglobin in the blood, the substance which also 

 makes our blood red. The worm is so transparent that 

 the full red color of the blood shines through the skin. 

 This case is interesting in reference to the question 

 whether the bright colors of many marine worms have 

 any useful purpose. Obviously haemoglobin has a 

 function in relation to respiration, and its red color may 

 have no particular significance as such. Did we not 

 know the physiological significance of haemoglobin, we 

 might be puzzled to offer any reason for the bright color 

 of the worm. The tubes of Polychaeta have as a basis 

 a secretion of the worms themselves, but frequently 

 particles of sand or fragments of shell are built in, much 

 as in the case of the tubes made by caddis-fly larvae 

 (page 273). The Serpulidce make calcareous (limy) 

 shells, suggestive of those made by mollusks. This re- 

 semblance is particularly striking in the genus Spirorbis, 

 the small tube shells of which are coiled like a snail. 

 The coiled shells of Spirorbis adhere to various objects, 

 and as they are hard they are easily preserved as fossils. 

 In strata many millions of years old, these small struc- 

 tures are found, apparently as well developed as those of 

 today. 



Polychaete gills are 'interesting structures, finely 

 branched, with the form of seaweed or feathers. Aquatic 

 insect larvae often show gills of various forms, having 

 the same function of absorbing oxygen from the water. 



4. The oligochaetes do not all live in the earth ; many Earthworms 

 inhabit fresh water. Thus the polychaetes and oligo- 

 chaetes divide the world between them, and there are 

 few places where one or the other may not be found. 

 Strangely, true earthworms appear to have been absent 



