Dr. Johnston on the Irish Annelides. ir>9 



each other so closely, that it is not, in some instances, easy to 

 decide what should constitute their permanent diagnostics ; 

 or to express, in a few apt words, the minute shades of dif- 

 ference in certain organs which seem to mark them as distinct 

 species. I am satisfied that, in this genus, the form of the 

 body of specimens preserved in spirits will afford no specific 

 character ; and that as little reliance can be placed on colour, 

 although this is perhaps more uniformly alike in living indi- 

 viduals. The number of segments is also, as Otho Fabricius long 

 ago remarked*, liable to considerable variation, both from 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 posterior 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 cha- 

 racter is one not to be neglected ; but, from the peculiar shape 

 of the jaw, I have sometimes found a difficulty in determi- 

 ning the exact number of these serratures ; and, in other in- 

 stances, 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 propor- 

 tions 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 se- 

 cond; (2) in the comparative lengths of the longest pair of ten- 

 tacular 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 



* " Cetemm numeravi sine respectu magnitudinis segmenta 56, 65, 76, 

 78, 86 in diversis; igitur de numero nil certi statiii posse patet : hunc cha- 

 racterem etiam quam maxime vacillare facile credat, cui rnutilatio et redin- 

 tegratio articulorum innotuit ; sub reintegrando enim articulo caudali pri- 

 mum accrescente, reliquis vero successive, a momento conspectus numerus 

 dependet." — Faun. Grcenl. p. 292. 



Ann. Nat. Hist. Vol. 5. No. 30. May 1840. n 



