18 NATURAL ARRANGEMENT OP INSECTS. 



possess processes or setse, which can be protruded or 

 withdrawn at pleasure, placed on each side of the 

 body, which serve the animal as feet. The construc- 

 tion of the mouth is excessively variable. In some 

 (Nereidce), it is lengthened into a proboscis furnished 

 with jaws : in the Serpulidte, or worm-shelfs, which 

 have a testaceous covering, there is an appearance of 

 two lips ; while in the leeches (Hirudmidce), there is 

 a prehensile cavity, supplied with parts which perform 

 the office of jaws. The vertical movements of these 

 parts, which so eminently distinguish vertebrate ani- 

 mals, is still observable in many of the Annelides; but 

 what particularly distinguishes the two groups from 

 each other, is the nervous system of these worms, which, 

 according to Cuvier and Savigny, is longitudinal, double, 

 and knotted, or ganglionic, like that of insects. In the 

 leeches, this similitude extends to the same number of 

 ganglions.* The connection of this group with the Verte- 

 brata and annulose animals has been before demonstrated. 

 We shall, however, in this place, cite the opinion of Mr. 

 MacLeay, who writes as follows : " That the Annelides, 

 thus lying between the two most perfect forms in 

 zoology, should be so inferior in the senses and powers 

 of locomotion to both, is certainly surprising ; " but, on 

 further reflection, we shall cease to think this anomalous, 

 and may in some measure even expect that the vital 

 powers of these beings ought to be influenced by the 

 circumstance, that Nature in them is leaving one plan of 

 construction, in order to adopt another which is totally 

 different. 



(15.) The affinity between these animals and the 

 fishes, through the medium of Gastrobranchus, is thus 

 stated by our author. u The common leech is a red- 

 blooded aquatic animal, which swims, like the lam- 

 preys, in an undulatory motion. Like those animals, 

 it has a circulation composed of veins and arteries : 

 it breathes, like them, by two rows of holes, which 



* Hor. Ent. 278. 



