190 



ZOOLOGY 



(Fig. 184) . From the mouth of the tul^e the head, with its tenta- 

 cles, may be protruded, but it quickly retracts from danger 

 and closes the opening of the tube, as a 

 marine snail docs its shell, by means of an 

 operculum or lid. 



We have now to consider some groups of 

 worms which differ from the annelids in not 

 having the trunk composed of a series of 

 rings. The first of these is the group of 

 nemerteans (Nemertini, Fig. 185). It in- 

 cludes chiefly marine animals, of some- 

 what flattened form and gTeat length, 

 even as great as thirty metres. They 

 protrude a long, slender proboscis. The 

 Ijody readily l)reaks up into a great many 

 pieces when the woi'ms are handled roughly, 

 but since each of the pieces may regenerate into an entire 

 worm, this behavior 

 is not self-destruction, 

 but self-preservation. 

 These worms are es- 

 peciall)' abundant in 

 the sand of the sea- 

 shore, although land 

 nemerteans occur. 

 The other two groujos 

 of unringed worms 

 have gained a parasitic 

 habit, and in conse- 

 quence have become 

 much modified in form 



Fig. 183. — Tube of 

 Cistenides slightly 

 enlarged. 



Fig. 1S4. — Scrpula tube on a bit of oyster shell 

 that is perforated by the boring sponge. The 

 tube lies in the centre of the figure. Nat. 

 size. Photo, by W. H. C. P. 



