ON THE BRITISH ANNELIDA. 209 



teen last of these setae, though beautified by the branchial tufts, are identical 

 in structure and action with the seven anterior. The setae consist of a long 

 rigid flattened shaft, supporting on either side long, closely-adjusted and 

 slender secondary setas, after the pattern of a joen. They are exactly penni- 

 form. Such a figure fits these organs most admirably for the practical work 

 which they are required to perform. Moving through a loose, yielding, 

 pulverulent soil, a smooth-edged seta would oppose too little resistance in 

 penetrating such a substance to enable the worm to make any forward 

 movement. Supplied, hovvever, with secondary setae, which expand and 

 separate from each other as the resistance of soil operates from the point 

 towards the root, each foot, although planted only into sand, becomes a firm 

 fulcrum on which the worm elongates its body and carries forward its snout 

 in its boring operations with considerable muscular power. 



A worm has lately fallen under the author's observation for the reception 

 of which he has ventured to constitute a new species under the name of Cly- 

 menoida arenicoida. The anterior part of the body in all essential characters 

 is closely similar to that of the Areiiicola. It is to this region of the body that 

 the setiferous feet are limited ; they amount to about seventeen in number ; 

 the setae are organized after the type of those of Arenicola ; the snout and 

 proboscis are also -analogous : there are no external branchiae. The poste- 

 rior four-fifths of the body is two-thirds embraced by strong muscular raised 

 rings, which support three lines of hooks (fig. 31), distinguished from all 

 those hitherto described in the Tubicola, in being sustained on a long stalk, 

 the whole forming the figure of S. These instruments are well calculated 

 to gain for the active and boring cephalic end of the body a firm basis of 

 movement, by securely anchoring the posterior portion to the walls of the 

 factitious tube in which the animal is generally found to be lodged. 



In these worms there are no specialized tactile organs under the form of 

 papillae or cirri. 



In the kindred genera, however, oi Euphrosyne, Hipponoe,Pleione, Chlceia, 

 while the feet are furnished with denser and larger brushes of setae, cirri 

 more or less developed are found always to exist. Of these genera few 

 illustrative species are found on the English shores. The feet are organized 

 more for moving readily through water than a solid soil. 



-Several elegant representative species of the genus Eunice frequent the 

 British shores. They are organized to move in underground tunnels. In 

 structure the feet present an adaptation to the wants of such a mode of life. 

 In the largest species oi Eunice, as E. antennata, below the branchiae, which 

 are most dorsally placed of all the parts of the feet, a tapering fleshy process 

 is observed, the office of which is evidently to guard the root of the branchia ; 

 it is tactile. Next to this element, on the ventral side, is situated a broad fan- 

 shaped cirrus, fitted well for the work of rowing through water or liquid mud. 

 This lamellated cirrus is perforated irregularly by canals, into which the fluid 

 of the peritoneal cavity freely enters; it is also protected by two bunches of 

 bristles, which are arranged on a vertical plane. At the extreme end of each 

 seta is remarked a joint-like break, at which the junction occurs between the 

 end (sabre-shaped) of the seta and its shaft (fig. 32). The union between these 

 two parts of the seta is not, however, an articulation. A true articulation 

 happens nowhere in the appendages of the Annelida. These quasi-articu- 

 lated setae act on a definite mechanical principle ; the extreme portion is 

 first firmly planted on the surrounding substance, on it then as a point 

 d'nppui the shaft revolves, and the body of the worm is moved forwards or 

 backwards, according to the direction of muscvilar action. The setae in 

 E. gigantea, of which no example has yet been found by the author on our 



1851. p 



