204 REPORT — 1851. 



rise or descend at will. The body of the Serpula is obviously distinguished into 

 two parts, of which one may be called the thoracic, and the other the abdomi- 

 nal. The former is provided with prominent feet, powerfully protrusile, and 

 a system of strong bristles (Plate VI. fig. 22, a), which during the movement 

 of the feet run to and fro in the axes of the feet. On the dorsal aspect of these 

 appendages, a row of microscopic hooks, marked by a minutely dark line, 

 extending transversely in part round the body, may be discerned (fig. 22, b). 

 It is by aid of these inimitable instruments that the worm grasps the interior 

 of the tube. They are wielded by means of long thread-like tendons, fixed 

 on artful mechanical principles, to the attached end of each hook. During 

 the action of the muscles, of indescribable delicacy, the hooks are pro- 

 jected to some distance beyond the plane of the surface on which they repose 

 "in the inactive state. These singular organs are formed after the pattern of 

 the common "bill-hook" of farmers, having the edge deeply notched into 

 teeth, directed downwards. Through the agency of these hooks the worm is 

 enabled to withdraw itself into its tube with extraordinary precision, rapidity, 

 and muscular force. In this movement, arithmetic would fail to compute the 

 number of these instruments, which is simultaneously extruded : their office 

 is exclusively that of pulling the animal back into its cell. Gifted with such 

 marvellous instruments, and alarmed by external danger, these elegant 

 Annelids will retreat with the rapidity of lightning. The advance move- 

 ment, in which it pushes itself out of its tube, is of course the reverse of the 

 former; but what is remarkable is, that this movement of emergence is per- 

 formed by means of organs quite distinct and diffei-ent from those used in 

 the act of retreating. In principle of action and construction, these instru- 

 ments are strikingly dissimilar — one is a pidling and the other is a pushing 

 machine. The setae or bristles, which are protrusile, are the pushing organs 

 (fig. 22, a). Nothing in nature is so perfect as the adaptation with which 

 these organules are fitted for the end in view. Each seta is composed of a 

 strong rigid and unresisting shaft, and an expanded shoulder, drawn out into 

 a point. On one side of this pointed shoulder may be remarked a double 

 row of serrations which are admirably calculated to catch against the surface 

 of contact, forming thereby a firm and fixed point of propelling force. 

 Computing the pushing force which each seta is capable of exerting, and 

 multiplying this amount by the number of setae in each foot, and this again 

 by the number of feet with which the worm is provided, a conception may 

 be formed of the aggregate of mechanical power with which the animal 

 executes its " march forwards." A similar calculation applied to the hooks, 

 will give a correspondingly prodigious resultant of power for retreat. The 

 mechanical principles thus imperfectly expounded, will correctly apply in 

 every particular to the instances of all tubicolous worms ; the hooks perform 

 one movement, the bristles another. Never before has the meaning of these 

 matchless instruments been differentially defined. The hooks and the 

 bristles are characteristic and distinctive of species. By one single micro- 

 scopic hook or seta, visible only under the highest powers of the microscope, 

 the naturalist may pronounce the species, and mentally reconstruct the indi- 

 vidual — no mean triumph for the science of observation I In the Serpuli- 

 dans, as in all the fixed tubicolous Annelids, the feet near the tail, acting 

 therefore at the bottom of the tube, are modified in structure with express 

 reference to the duties of mopping, sweeping, scraping, and wiping the caecal 

 end of the habitation. The organs for the discharge of these necessary 

 household duties will be afterwards described more at length in the Amphi- 

 tritans. 



The SabellcB, like the Serpulidce, are tubicolous ; the tubes of the SabelUs 



