124 
STARFISH GALLERY. 
The Oligochata are represented by the common Earthworm, the 
influence of which in the formation of mould and in the general 
ploughing of the soil was carefully investigated by Mr. Darwin ; 
and by the little Tubifex rivulorum (Bloodworm), which owes both 
its red colour and its ability to dwell in mud, which is so poor 
in oxygen as to be unfit for respiration, to the same chemical 
compound as that which gives the red colour to our blood and 
carries the oxygen of respiration all over the body. 
The Hirvdinea, or Leeches, are often said to be distinguished 
from the Chcetopoda by the absence of bristles , but, as a fact, 
Acanthobdella (Figs. 21 and 22) has very well marked bristles. They 
Fig. 21. 
Fig. 22. 
Acanthobdella : e, eyes ; ch, chaetse ; s, sucker. 
always have a sucker at the hinder end of the body by which they 
are attached to their prey ; they are found in fresh water (Piscicola), 
on sea-fishes (as Pontobdella), or in moist places, as the Leech 
(Eirudo). The last-named has three jaws, armed with as many 
