224 ANATOMY AND PHYSIOLOGY 



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 charac- 

 terised, that description will most probably suffice perma- 

 nently 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 characterising 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 com- 

 parison 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 sometimes reversed by 

 drying ; thus the surface smooth in the live state become 

 villous when dried. Inconspicuous oscula become con- 

 spicuous when contracted and dry, and conspicuous 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. 



