230 GENERAL CHARACTERS OP ANNELIDA. 



water, serving in some degree as oars by which it is propelled. 

 In some instances, indeed, we find the tufts replaced by flattened 

 plates, which are specially adapted for this last purpose. Where 

 there are no locomotive appendages, the extremities of the body 

 are usually furnished with suckers, which give important assist- 

 ance in locomotion, as in the well-known Leech. But in one 

 tribe of this class, the animal, in its adult form at least, enjoys 

 very little power of locomotion, being confined within a shell, 

 which it constructs for itself, and which is attached to some 

 solid support. 



906. The first segment, which constitutes the head, is usually 

 provided with one or more pairs of imperfectly-formed eyes ; 

 and also with several appendages analogous to the cirrhi of the 

 other segments, which are considered as antennae or tentacular 

 cirrhi. The mouth, which is situated on the lower side of the 

 head, ia constructed on a very different plan in the several divi- 

 sions of the class : being sometimes furnished with one, two, or 

 three pairs of hard horny jaws, with toothed or pointed edges ; 

 sometimes having a sort of trunk, which can be pushed out or 

 tr drawn in (Fig. 624), and which 



bears a pair of small tooth-like 

 jaws at its extremity ; and some- 

 Mines being situated in the centre 

 of a flattened sucker, and armed 

 with an apparatus of little saws, 



FIG 624 HEAD AND TRUNK OF GLV- ,, T > /-.-,. /? i \ 



c, anterior portion of the as m the Leech ( Fl g- 631 ) 



body ; t , head ; tr, trunk ; 6, opening alimentary canal is usually simple 



of the mouth ; m, m, jaws. . . ' 



m its form, passing in a straight 



line from one end of the body to the other; and not exhibiting 

 any distinction of stomach or intestine. It is often furnished 

 with a number of little saccular appendages, placed along the 

 greater part of its length ; these are probably 'secreting organs, 

 as we do not find any others which can be regarded as having 

 that character. Many of the Annelida are remarkable for the 

 red colour of their blood ; this colour is not given, however, by 

 red corpuscles, but exists in the liquor sanguinis (Amii. PHY- 

 SIOL. 229). Sometimes, however, the blood has rather a 



