ANNELIDA TERRICOLA. 



become gradually diminished in size, so as to terminate, when 

 the worm is fully stretched out, in a fine point. But there is 

 another circumstance in which the external anatomy of the 

 terricolous Annelides differs materially from what is seen in 

 the suctorial Abranchia. In the latter the tegumentary seg- 

 ments are quite naked upon their outer surface, but in the 

 Lumbrici, now under consideration, every ring, when exam- 

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

 spines or prickles. These, indeed, are so minute in the earth- 

 worm, 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 circumstance which arises from their 

 hooked form, and from their points being all turned towards 

 the tail. These differences between the external structure of 

 the suctorial 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 is evident on carefully 

 observing the manner in which the earth-worm bores its way 

 through the soil in which it lives. The attenuated rings in 

 the neighborhood 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 recurved spines appended to the 

 different segments. The hinder parts of the body are then 

 drawn forward by a longitudinal contraction of the whole 

 animal ; a movement which not only prepares the creature for 

 advancing further into the soil, but, by swelling out the ante- 

 rior segments, forcibly dilates the passage into which the head 

 had been already thrust. The spines upon the hinder rings 

 then take a firm hold upon the sides of the hole thus formed, 

 and, preventing any retrograde movement, the head is again 

 forced forward through the yielding mould, so that, by a repe- 

 tition of the process, the animal is able to advance with the 

 greatest apparent ease through substances which it would at 

 first seem impossible for so helpless a being to penetrate. 



The alimentary canal of the earth-worm is straight and very 

 capacious. Its great size, indeed, is in accordance with the 

 nature of the materials employed as food, for it is generally 

 found distended with earth ; and, indeed, by the older physi- 

 ologists, these creatures were generally regarded as affording 



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