4 A HISTORY OF RECENT CRUSTACEA 
to one of these, they are ‘AquaticArthropoda, which breathe 
by means of gills. They have two pairs of antenne, 
numerous paired legs on the thorax, and usually also on 
the abdomen.’ This is compendious and useful. The 
statements clearly exclude all the other classes of the 
Arthropoda. ‘They are also widely applicable among crus- 
taceans ; yet of these animals there are some which are 
not aquatic, some which have no gills, some which have 
not two pairs of antennee, and some in which the ‘ paired 
legs on the thorax’ are not numerous. 
A different definition was given by Professor Alphonse 
Milne-Edwards in 1860, according to which the class of 
Crustacea comprises ‘all the segmented animals with bran- 
chial or cutaneous respiration, in which the body is pro- 
vided with jointed limbs, whether permanent or transitory.’ 
The Insecta and Myriapoda breathe by means of the air- 
tubes called tracheze; most of the Arachnida by means either 
of tracheze or pulmonary sacs known as fan tracheee. From 
all these, therefore, the definition separates the Crustacea in 
a satisfactory manner, even though some terrestrial Crus- 
tacea combine tracheate with branchial respiration. There 
are, however, some subordinate members of the Arachnida, 
and the whole class Pycnogonida, in which the respiration is 
dependent on the surface of the body and not on any special 
organs. Asit is only in recent years that the Pycnogonida 
have been constituted an independent class, it was no 
fault of a definition framed in 1860, that it included them 
among the Crustacea, to which they were then supposed to 
belong. They are in fact separated by many characters, one 
of which is the possession of a proboscis, which is supposed 
to have originated in the coalescence of the upper lip and 
the mandibles. So far as is known, they are all marine 
animals. On the other hand, those Arachnida which have 
surface-respiration are apparently all air-breathers. ‘To 
meet all existing requirements, then, the definition of the 
Crustacea may be framed in the following manner :— 
They are Arthropoda without terminal proboscis, with 
respiration branchial or cutaneous, the latter only aquatic. 
It is not to be expected that any legitimate definition 
