130 NAIS. 
practical observer in composing original treatises on rare subjects, and 
fillmg numerous plates with a profusion of the finest representations 
possible to be executed of them. Not only are the subjects themselves 
of the rarest occurrence, insomuch, that the observations on a single 
specimen of to-day may never be corroborated from want of opportunity, 
because a second specimen may not be found for ten, nay for twenty years 
afterwards. Such a specimen may not be perfect, neither may it survive, 
or a superior artist may not be every instant at command, and hence we 
may easily conclude how little of the whole nature of an animal can be 
disclosed, even under the most favourable circumstances, on seeing an 
object twice. Hence, I cannot hesitate to affirm, that no single indivi- 
dual is capable of composing the complete and entire history of any one 
animal, or race of animals, for this must be the work of many. Should 
any one chance to confirm the notice of peculiarities ascribed to any ani- 
mals in this volume, I hope that naturalists will permit him to enjoy his 
own share of the merits his skill and patience may deserve,—recollect- 
ing that I make no claim to priority. How could I, unless I were fami- 
liar with the pursuits and investigations, present and past, of all the 
world. The best promotion of the science of natural history, and the 
justest correction of observations, will be effected by the accurate delinea- 
tion, aided by the description of living subjects in the vigour of life. 
I have had little opportunity of studying the aspect and habits of 
the hispid worms, whereof two, belonging to the genus Nais, are inhabi- 
tants of the fresh-waters of Scotland. 
But one principal reason for introducing them, originates from the 
desire of proving this fact, because the disappointment attendant on 
long and frequent research, made me begin to doubt whether either of 
them could be numbered in the Fauna of this country. 
I. NAIS. 
§ 1. Nats Lacusrris.—Plate XVII. Figs. 1-5. 
Length an inch ; body round ; extremities obtuse ; the anterior 
