40 History of British Entomostraca. 
Ostrapodes corresponds with Latreille's group Ostracoda ; the Cope- 
podes, with his Carcinoida, the Cladoceres, with his Cladocera, and 
the Phyllopodes corresponds with his second principal section, the 
Phyllopa. In this arrangement, therefore, the two last legions of 
the Maxilles, the Entomostraces and Branchiopodes, correspond ex- 
actly with Latreille's first order, the Branchiopoda ; and form a 
very natural group ; whilst the second order, the Paecilopoda, is se- 
parated altogether from the Entomostraca, properly so called, and 
correspond with M. Milne-Edwards's sub-classes, Suceurs and Xy- 
phosuriens. This arrangement I shall adopt in the remarks which 
follow, and my Catalogue of British " Entomostraca" shall thus 
be confined to Edwards's two legions Entomostraces and Brachio- 
podes, or Latreille's Branchiopoda. 
The animals of this group, when noticed in their native*habitats, 
may be seen to have their branchiae constantly in motion, their ac- 
tion being seldom interrupted. One chief use, therefore, of these lit- 
tle insects, in the economy of nature, maybe, as Muller says,* to ven- 
tilate the water day and night ; and as they chiefly reside in stand- 
ing pools, they may thus be of great use in preserving them from 
becoming soon putrid. As this may be considered one of the bene- 
fits conferred by these insects, it may be useful to know the evils to 
man they may be likely to produce. Though they are most abundant 
in stagnant water, they yet occur in considerable numbers in the 
purer sorts of water that serve as our common drink, and may fre- 
quently be seen in considerable quantity even in the drinking water 
of London, Edinburgh, &c.; and Muller asserts very gravely, that, as 
we thus drink them alive and with their eggs, he would not be sur- 
prised were we to discover them some day in the human intestines.t 
" The time," he says in another place,J " is at hand, when the causes 
of disease shall not only be sought after in the air, in our method of 
living, &c. but in the incautious use of waters, often abounding in 
innumerable animalcules." According to Muller and Straus, these 
insects live upon vegetable matter and not upon animals ; and the 
former, in an experiment he instituted, says, that in keeping a num- 
ber of species, such as the Daphnia pennala and longispina, Cypris 
slrigata and pilosa, Lyjiceus sphcericus and Cyclops quadricornis, in 
the same \vater from the 24th July to the 22d January, during 
which time the water had evaporated from a depth of five inches to 
that of one, he frequently subjected small quantities of this water 
to the microscope, and was never able to discover any animalcules 
in it upon the most attentive examination, though the intestines of 
* Entomostraca, p. 8. f Ibid. p. 33. \ Ibid. p. 12. Ibid. p. 7. 
