NEREIS. 147 



age and from mutilation ; for if the posterior segments have been 

 lost by accident, they are indeed again renewed, but not in their 

 original numbers or size ; and moreover it is often very difficult to 

 count the segments from the minuteness and crowding of the pos- 

 terior ones. The pattern after which the prickles of the proboscis 

 are arranged varies in some species, but it is almost impossible to 

 define those variations in words, and the character fails us in the 

 nearest allied species, where only it is required*. Such is also the 

 case with the number of serratures along the falcate edge of the jaws, 

 though the character is one not to be neglected ; but from the pecu- 

 liar shape of the jaw, I have sometimes found a difficulty in deter- 

 mining the exact number of these serratures ; and, in other instances, 

 have had a doubt whether one or two of them, from their obsoleteness, 

 ought to be reckoned. I place little value on any differences in 

 the shape of the head, or in the proportions between the palpi and 

 antennae ; but a specific character, it appears to me, may be justly 

 founded on differences (1) in the proportion of the first or post- 

 occipital segment to the second ; (2) in the comparative lengths of 

 the longest pair of tentacular cirri ; but (3) principally in the variety 

 exhibited by the lobes and appendages of the feet. Every foot, let 

 it be remembered, consists of a superior and an inferior cirrus, three 

 papillae presumed to be branchial, and two tubercles armed with 

 compound bristles, — the superior tubercle being always situated 

 between the dorsal and second papillae, and the inferior tubercle be- 

 tween this and the ventral papillae. On these particulars I will 

 endeavour to define the British species before me ; and I trust that, 

 with the designs which illustrate the specific characters, the student 

 will now be able to determine, with comparative ease and certainty, 

 such of them as he may meet with in his researches. 



1 . N. brevimana, post-occipital segment not much longer than the 

 second, the tentacular cirri once and a half or twice the length of 

 its diameter ; feet with subequal parallel lobes, the inferior connate, 

 with the setigerous branch on the posterior feet ; terminal piece of 

 the bristles smooth ; cirri very short, not reaching the apex of their 

 lobes ; jaws with eight denticulations. Length 3". 



Nereis brevimana, Johnston in Ann. Nat. Hist. v. 170 (1840). 

 Nereis Sarsii, Rathke in Nov. Act, Curios. Ccesar. xx. 161 (1843) 

 tab. 8. f. 6-8. 



Hab. The littoral region. 



Besc. About 3 inches long, and about the size of an earthworm 



have purposely abstained from specifying the number of segments as a leading 

 feature. It appears to me, in as far as hitherto determined, that a frequent, if not 

 a continual, evolution of new articulations is advancing, so as to preclude calcu- 

 lation of any precise number as belonging to a species." — Sir J. G. Dalyell, Pow. 

 Great, ii. p. 163. 



* Blainville believes that these prickles will afford the best specific character. 

 —Diet, des Sc. nat. Ivii. 472. 



l2 



