History of British ILntoinostraca. 35 
In stating my opinion of the manner in which we may explain 
the phenomenon of a " tree within a tree/' without the necessity of 
referring to the theory of Du Petit Thouars, I do not wish to be 
understood as asserting this theory to be incorrect. If it is to stand, 
it will doubtless be one day established upon clear and conclusive 
experimental proof; but in the meantime it is right for us to be 
cautious in admitting the explanation of any phenomenon upon the 
supposition of the theory being true, which may as easily be ac- 
counted for upon other and well established principles. 
III. The Natural History of the British Entomostraca. By 
WILLIAM BAIRD, Surgeon. 
THERE is no order of insects to which so little attention has been 
paid by the naturalists of Great Britain, as the Entomostraca, and 
there are few, if any, that are more deserving of attentive study. 
The external appearance of these little interesting creatures, viz. 
insects enclosed in a testaceous covering, is enough of itself to excite 
the curiosity, and rouse the interest of the naturalist. A great num- 
ber of them, indeed, are so like shells, that a person ignorant of the 
inhabitants would not fail at first sight to call them so ;* and the 
very singular phenomenon of animals belonging to the class of in- 
sects, being so covered with a shell, has supplied to Miiller the 
name of Entomostracon, a term derived from two Greek words, sig- 
nifying " an insect with a shell ;"t a name which this order of in- 
sects has hitherto maintained, and which has been almost univer- 
sally adopted by naturalists from Miiller's time to the present day. 
Previous to the appearance of Miiller's work, the few species which 
were known, were arranged under one genus, the genus Monocu- 
lus and they were so called from their possessing, or appearing to 
possess, only one eye. Schoeffer appears to have proposed the name 
of Branchiopodes for them, from their feet possessing branchial ap- 
pendages; and Miiller says he would have preferred this name to 
that of Monoculus, were it not that several species wanted these or- 
gans. The species, however, which he mentions are either the Nau- 
plii and Amymones, not aware that these were but the imperfect 
young of the genus Cyclops, or the Cythere, which possesses bran- 
chial appendages attached to the jaws instead of the feet. These 
facts, however, he was not aware of, and therefore he preferred to 
either of the above names, that of Entomostracon, a name which I 
* Miiller. f Entomostraca, p. 2. 
