SECT. v. TUBICOLA. 153 



pencil like a fan. They are very slender, bent at the 

 tip, and so transparent that they look like threads of 

 spun glass ; the worm thrusts them out and draws them 

 in with extreme rapidity. 



A blood-red Nais lives in burrows in the mud at the 

 bottom of springs and pools in immense multitudes ; 

 large tracts of the mud of the Thames are red with a 

 species of them ; half of their bodies stretched out of 

 their burrows maintain a constant oscillating motion on 

 its surface, but, like the earth-worm, they instantly 

 shrink into their burrows on the least alarm. They 

 have no respiratory organs ; but their blood is aerated 

 through their skin., which is so transparent that, with a 

 microscope, the whole of the internal structure, the mo- 

 tions of the liquids, and the particles they contain are 

 distinctly visible. The blood acts the part of internal 

 gills, by aerating the colourless liquid contained in a set 

 of vascular coils surrounding the organs of digestion. 



The Tubicola are marine worms, forming the third 

 order of Annelida, according to the system of M. Milne- 

 Edwards. They live in tubes, either of a shelly cal- 

 careous substance, which forms naturally on the tena- 

 cious mucus of their skins, or in tubes artificially con- 

 structed by themselves of sand and particles of shell 

 glued together. All the Tubicola can protrude their 

 gills and the anterior part of their bodies, and some 

 can leave their tube and return to it again. These 

 worms, which form beautiful objects for the microscope, 

 have ringed bodies with tubular bristled feet, and respi- 

 ratory organs or gills fixed either on the head or near it. 

 They have an alimentary canal loosely attached to the 

 ventral wall of the body, and two systems of circulating 

 liquids, one red, the other colourless. In the Tubular 

 Annelids the principal organs of respiration are the con- 

 tractile plumes on the head. 



In the Terebella there are distinct organs for the 



