ON THE BRITISH ANNELIDA. 213 



The question of how far they are capable of fulfilling a lespiratory function 

 was formerly considered. That they are of great use in locomotion and 

 sensation admits of no doubt. The inferior cirrus in every species is short and 

 unjointed. Between these cirri is placed the setiferous process. No single 

 anatomical character is calculated to suggest so much with reference to the 

 habits and manners of the Annelidae as that derived from the structure of 

 the setae. Moreover, these characters are subject to no variation. They are 

 in the Annelids as fixed in their constancy as the teeth are in the higher 

 animals ; they form a magic key which unlocks the hidden secrets of the 

 ceconomy and organization of these worms. In S. armillaris, moving over 

 the hard surfaces of stones and shells, the setse are furnished only with a fine 

 shape-edged penknife-like articulated piece, well adapted to fit itself into 

 cracks and crevices, while the extremity of the supporting shaft is pointed 

 sharply, with an obvious view to catch the surface against which it is applied 

 during progression. Faithful to the inviolable law of "appropriate means 

 to intended ends," it may be remarked that in kindred species, differing in 

 scarcely any other respect than that of its habitat from the former, ingenious 

 nature, by a trifling modification in the figure of the setae, enables S. pro- 

 lifera to creep with ease and steadiness over the smooth and slippery surfaces 

 of the glutinous Algae. This object it accomplishes in the most artistic 

 manner, merely by curving downwards the extreme end of the articulated 

 portion of the seta. This is done twice ; so that each piece is furnished 

 with two minute hooks on the inferior side, by which, when planted into the 

 soft vegetable tissue, the worm is enabled to secure itself ' in place' with 

 strength and certainty. The author has recently discovered that in twct 

 species oi MyrianidcB which affect similar situations in Algae, the setas exhibit 

 a structure which is designed to secure the same object, which are no less 

 marvellously adapted to the exigences of the case than the feet of the house- 

 fly or those of the Polar Bear. 



In Psamathe fusca there prevails an arrangement of the setae which is 

 the exact reverse of the formation just described in the former species. In 

 this Annelid, which also lives on algae, the setae are constructed for pushing, 

 and not for pulling and fixing. The articulated piece is serrated in the 

 direction of its point. These teeth are well appointed for catching in the 

 soft surface while they are being protruded from their sheath. This worm 

 therefore accomplishes its locomotion by pushing, the feet being directed 

 backwards. The true Syllidoe walk by creeping, pulling themselves for- 

 wards, the feet being first thrown in advance of their respective rings. 



The genus Nerine or Spio exemplifies, in the disposition and actions of its 

 tactile and motive appendages, the principle which Paley has matchlessly 

 expounded, that every mechanical perfection realized in the animal organism 

 is attained in strict conformity with the laws of physics, illustrating the 

 subordination of the principles of one kingdom of creation to those of an- 

 othei*. The Nerine move with remarkable facility through sand or shingle. 

 They progress in water slowly and awkwardly, oscillating from side to side 

 without making scarcely any advance. This is the necessary result of a 

 bodily structure suited in none of its mechanical appliances for progression 

 in such an element. In loose sand, however, " the medium " in which they 

 are designed by nature to revel, they display the most vigorous agility. By 

 means of their pointed snout they burrow with great skill and effect, carrying 

 passively at the sides their long manual appendages. The mechanical 

 constituents of the feet are peculiarly adapted to aid in this operation. The 

 dorsal foot, situated below the branchia, is composed of a fan-shaped cirrus, 

 "the plane of which is vertical, that is, parallel with a line carried from the 



