ANNELIDA. 



means provided for enabling it to overcome difficulties apparently 

 insurmountable by any animal unless provided with limbs of extra- 

 ordinary construction, and possessed of enormous muscular power. 

 In the mole and the burrowing cricket we at once recognise in 

 the immense developement of the anterior legs a provision for 

 digging, admirably adapted to their subterranean habits, and cal- 

 culated to throw aside with facility the earth through which they 

 work their way ; but in the worms before us, deprived as they 

 appear to be of all external members, feeble and sluggish even to 

 a proverb, where are we to look for that mechanism which enables 

 them to perforate the surface of the ground, and to make for them- 

 selves, in the hard and trodden mould, the pathways which they 

 traverse with such astonishing facility and quickness ? 



(245.) The structure of the outer fleshy integument of the earth- 

 worm resembles in every respect that of the leech already described, 

 both in the annular arrangement apparent externally, and the disposi- 

 tion of the muscular strata. The suctorial discs, however, which 

 in the leech formed such important instruments of progression, 

 are here totally wanting ; and the annular segments of the body, 

 as they approach the anterior extremity, become gradually dimi- 

 nished in size, so as to terminate when the worm is fully stretched 

 out in a fine point, near the apex of which is the opening of the 

 mouth. But there is another circumstance in which the external 

 anatomy of the terricolous Ann elides differs materially from what 

 we have seen in the suctorial Abranchia : in the latter, the tegu- 

 mentary segments were quite naked upon their outer surface ; but in 

 the Lumbrici, of which we are now speaking, every ring, when ex- 

 amined attentively, is found to support a series of sharp retractile 

 spines or prickles ; these, indeed, are so minute in the earthworm, 

 that, on passing the hand along the body from the head backwards, 

 their presence is scarcely to be detected by the touch, but they are 

 easily felt by rubbing the animal in the opposite direction ; a cir- 

 cumstance which arises from their hooked form, and from their 

 points being all turned towards the tail. These differences be- 

 tween the external structure of the suctorial and setigerous Abran- 

 chia, minute and trivial as they might seem to a superficial ob- 

 server, are however all that are required to convert an aquatic 

 animal into one adapted to a subterranean residence, as will be 

 evident to any one who observes carefully the manner in which the 

 earthworm bores its way through the soil in which it lives. The 

 attenuated rings in the neighbourhood of the mouth are first insi- 



