72 Invenebrata* 



held in solution in the sea water. There are also seg- 

 mental tubes opening one on each side of each seg- 

 ment, and sometimes the eggs, which are produced 

 within the body, escape through these canals. The 

 chain of nervous ganglia is also well developed. Some 

 worms secrete a glutinous material from their surface, 

 which cements together sand-grains and other foreign 

 bodies into a tube wherein the animal lives. Other 

 worms secrete from their surface calcareous matter 

 which makes up a tube as a dwelling-house, in which 

 ihe animal is permanently contained. Such forms 

 have the gills developed only on the foremost seg- 

 ments of the body, and have the dorsal and ventral 

 oars of all the other joints rudimentary ; but they 

 possess tentacle-like, branching processes about the 

 head. Of these the common Serpula, whose white 

 calcareous snake- like concretions are so common on 

 the stones and shells on the sea shore, and the 

 Spirorbis, whose minute white whorled shells dot the 

 surface of the shore-tangles, are examples. 



A few worms are phosphorescent ; many others, 

 like the sea-mouse, are clad with iridescent scales and 

 bristles. 



The common earthworm has only eight short re- 

 curved bristles on each segment, arranged two in the 

 place of each oar, and not elevated on processes of 

 the surface. The body is closely ringed and tapers 

 from the middle forwards to an acute point in front. 

 The armature of each ring can easily be felt by draw- 

 ing the body of a worm between the fingers from tail 

 to head, although they are scarcely to be detected 

 when we feel the body in the reverse direction, In 



