72 Invcrtebrata. 



There are also segmental tubes opening one on each 

 side of each segment, 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 cal- 

 careous matter which makes up a tube as a dwelling- 

 house, in which the animal is permanently contained. 

 Such forms have the gills developed only on the fore- 

 most segments 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, 

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

 bristles. 



The common earthworm has much smaller and 

 fewer bristles, which are in the form of recurved 

 hooks, not elevated on stumpy processes of the sur- 

 face. The body is closely ringed and tapers from the 

 middle forwards to an acute point in front. Each 

 ring bears its armature of hooks, which can easily be 

 felt by drawing 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 beginning to burrow, the worm lengthens 



