1130 
DE. J. S. BOWEEBANK ON THE ANATOMY 
marginal cavities, if present, are not always distinguishable; and the external and 
internal defensive organs are, either one or both of them, frequently absent. 
Specific characters should always be of a positive nature, such as the presence and 
form of particular spicula or other organs. It is a great mistake in writing specific 
descriptions, to make the differences between species to consist of one or two striking 
essential characters only. Such a practice may answer tolerably well when there are 
hut two or three species of a genus known ; but it frequently occurs when new species 
are found, that they also have the most striking essential characters of the previously 
known ones equally strongly developed. Much confusion is thus likely to occur from 
this paucity of description ; whereas, if the whole of the essential characters of each 
species be carefully investigated and accurately recorded when it is first characterized, 
that description will most probably suffice permanently to distinguish it as a species, 
however numerous the subsequently discovered members of the genus may be. 
Differential characters should never be intermingled with essential ones in character- 
izing the species. They should be reserved for the amplified history ; and here they are 
of much value, as they lead to the relative consideration of two or more nearly allied 
species, and frequently assist the student in their discrimination when the essential 
characters are minute or somewhat obscure. 
In the description of species the adjectives long, short, stout, slender, &c., must 
always be understood as in comparison with the congenerous organs of the species under 
consideration, and not as in relation to any fixed standard of size. 
In the description of a new species it should always be stated whether the characters 
are given from a dried specimen, or whether from one fresh from the sea, as it frequently 
happens that many of the natural characters become completely obliterated and some- 
times reversed by drying ; thus the surface smooth in the live state become villous when 
dried ; inconspicuous oscula become conspicuous when contracted and dry, and conspi- 
cuous oscula are often destroyed by desiccation ; and so on with other characters. It is 
therefore absolutely necessary that the condition of the specimen should be stated along 
with its description. 
On the Preservation and Examination of the Spongiadoe. 
The greater portion of specimens in natural history may be readily examined and 
their species determined in the field ; but this is rarely the case with the Spongiadse. 
It becomes necessary therefore to preserve them in such a manner as to efiectually 
retain their natural characters for examination at some future period. Small specimens 
may be preserved in spirit of wine ; but this destroys their colour. If they are not likely 
to be permanently lodged in the cabinet immediately, it is better that they should be 
laid on blotting-paper, or a soft cloth, to absorb as much as possible of the water from 
within them, and then dry them rapidly before a fire, or in a slack oven, without any 
previous washing in fresh water. By this mode they retain a sufficient amount of 
moisture and flexibility to allow of their being handled and operated on for examination 
