218 REPORT 1851. 



appendages formed by a mutation of the superior cirri, and constituting a 

 general terminal style. The setae are serrulated at their extremities, and 

 formed for pushing (figs. 45 and 46). 



The Abranchiate Annelids, although destitute of outward organs of respi- 

 ration, are not all unprovided with external organs of locomotion. The 

 familiar Lumbricus exemplifies this statement. Tlie Eai-th-worm in its move- 

 ment displays great force of muscle. Its integumentary system is a complex 

 web of strong circular and longitudinal muscular fascicles. The whole force 

 of this machinery is brought to bear in progression on the stiff' advantageously 

 curved spines, which are planted in form of feet, in a double row on either 

 side, on the ventral aspect of each segmental annulus. These setae present the 

 form of the italic/ (fig. 47), only two of which exist in each foot. To the 

 inserted extremity of each seta an appropriate system of muscles is attached. 

 To the free end minute flexible hairs are added, the office of which is evi- 

 dently to prevent the gathering of dirt and earth on the part. These setae 

 will actually penetrate a deal board ; for if the path of a worm on the fine- 

 polished surface of a deal board be examined with the microscope four 

 series of minute perforations may be detected. In Lumbricus these setae 

 begin at the fourteenth annulus from the head. In the act of burrowing 

 into a fresh surface, as when the worm, irritated by the observer, strives to 

 return to the earth, the foremost feet are firmly planted in the ground, the 

 head retracted, and then thrust forwards with extraordinary force. It is 

 manifest from tlie mechanism of this operation, that feet placed nearer the 

 head, if such were the case, would rather obstruct than aid the burrowing 

 and thrusting power of this part. A sioollen wave of contracted rings may 

 be seen travelling from the tail towards the head while the worm is thus 

 engaged, showing the intermittent and successive manner in which the 

 " labour of contraction " falls on every segment of the body. While running 

 through its subterranean vaults, this Avorm continually plunges " into fresh 

 fields and pastures new," swallowing almost with the voracity of the Arenicola 

 the very substance of the earth which gives it shelter. 



The little lively Naides, though terricolous in habits like the Earth-worm, 

 are very dissimilar in organization. Of the genus Nais there are several 

 marine and terrestrial and freshwater species. In all, the mechanical ele- 

 ments of the feet conform to a common type of structure. A strong seta, 

 forked at the extremity for pushing (figs. 48 and 49), accompanied by one 

 or two plain hairs much longer than the former, composes the foot. This is 

 only a provision for enabling the worm to run up and down its soft tube, 

 which in N.filiformis is constructed in bottom-mud of freshwater pools, and 

 in the marine species in the substance of the hardest calcareous rocks. In 

 these latter Annelids, as will be afterwards shown, there is furnished no 

 special boring apparatus. The genus Chjmene is remarkable for the cha- 

 racter of its tube. The worm, in the instance of nearly every species, con- 

 structs a tube to envelope only the middle of the body ; the head and the 

 tail projecting beyond its limits. The worm, however, can by the retraction 

 of the two extremes of its body, conceal and protect itself within its tube. 

 These, like all other tubicolous Annelids, are provided with special mecha- 

 nical organs (hooks) for moving up and down its cell. In this genus the 

 hooks are supported on long stalks, and placed only at long distances on the 

 body. Each row of hooks comprehends hundreds of individual hooks, so 

 closely packed are they and microscopically minute. 



The genera Hirudo and Linus are wholly destitute of external appen- 

 dages, even for mechanical purposes. The former progress on the rings 

 immediately, the latter by the undulatory swelling and tightening of sue- 



