ABRANCHIA TERRICOLA. 229 



tions of an Earthworm when busied in burying itself in the earth, must 

 have been struck with the seeming disproportion between the laborious 

 employment in which it is perpetually engaged, and the means provided 

 for enabling it to overcome difficulties apparently insurmountable by any 

 animal unless provided with limbs of extraordinary construction and 

 possessed of enormous muscular power. In the Mole and the burrowing 

 Cricket we at once recognize, in the immense development of the anterior 

 legs, a provision for digging, admirably adapted to their subterranean 

 habits, and calculated 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 whereby they are 

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

 selves, in the hard and trodden mould, the pathways that they traverse 

 with such astonishing facility and quickness ? 



(597.) The structure of the outer fleshy integument of the Earthworm 

 resembles in every respect that of the Leech, already described, both in 

 the annular arrangement apparent externally, and the disposition of the 

 muscular strata. The suctorial disks, however, that 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 diminished 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 wherein 

 the external anatomy of the terricolous Annelidans differs materially 

 from what we have seen in the suctorial Abranchia : in the latter, the 

 tegumentary segments were quite naked upon their outer surface ; 

 but in the Lumbrici, of which we are now speaking, every ring, when 

 examined 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 pre- 

 sence is scarcely to be detected by the touch, but they are easily felt by 

 nibbing the animal in the opposite direction, a circumstance arising 

 from their hooked form, and from their points being all turned towards 

 the tail. These differences between the external structure of the suc- 

 torial and setigerous Abranchia, minute and trivial as they might seem 

 to a superficial observer, 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. The attenuated rings in the 

 neighbourhood of the mouth are first insinuated between the particles of 

 the earth, which, from their conical shape, they penetrate like a sharp 

 wedge ; in this position they are firmly retained by the numerous re- 

 curved spines appended to the different segments : the hinder parts of 

 the body are then drawn forwards by a longitudinal contraction of the 



