STRUCTURE OF SPIRORBIS. 275 



minute, seldom larger than pins' heads ; and instead 

 of being, like those of the Serpula, of an irregular 

 twisted figure, they are rolled upon themselves in a 

 regular spiral form, with one of the flat sides firmly 

 cemented to the surface of the object to which the 

 creature attaches itself. 



When magnified (PI. V. fig. 5), the neck of the 

 Spirorbis is found to be furnished with a perfect 

 operculum or lid, formed as in Serpula by a peculiar 

 development of one of the tentacula in the vicinity 

 of the mouth, which answers the purpose of a little 

 door, and likewise with an elegant apparatus of 

 branchial plumes. 



A little reflection will at once show us why the 

 respiratory organs of these tube-inhabiting worms are 

 situated in so remarkable a position, being invariably 

 attached to the neck of the animal, and not, a% in 

 other races of Annelidans, distributed 011 various parts 

 of the body. Had the breathing surfaces been placed 

 in any other situation, it is evident that while the 

 worm remained in its shell, the water could never 

 have obtained free admission to them, and conse- 

 quently respiration would have been impossible ; 

 whereas, by the elegant arrangement adopted, the 

 simple raising of the operculum which closes the ori- 

 fice of the shell like a lid, or the slightest protrusion 

 of the creature's head, suffices to ensure the due 

 aeration of the blood, without unnecessarily exposing 

 the little worm to danger. 



