264 
ZOOLOGICAL LITERATURE. 
Darwin he came to the conclusion that as the form which he 
terms the Nauplius form is the earliest stage of development 
known in an independent animal, the earlier Crustacea must at 
some period of their development have passed through a similar 
stage ; that it was his conviction on this point which induced 
him, when he saw in the field of his microscope a young Nauplius, 
to trace out its history, and thus he has been enabled to de- 
monstrate the earliest larval condition of the genus Petueus ; 
and that it was this circumstance which turned the scale of his 
judgment in favour of Darwin^s theory. 
He argues that the uniform number of somites in Malaco- 
stracous Crustacea and the invariable difference of the equipment 
of the last seven from the others, are evidences of their descent 
trom one common parent. He thinks that a similar argument 
may be based on the uniformity of the marine Decapoda passing 
through a Zoea of larval condition, and contends that Tanais 
among the Isopoda possesses many of the peculiarities that 
belong to the Zoea stage, and therefore may be said to represent 
the Zoea among the Edriophthalma, particularly in the manner 
of its breathing. 
We certainly do not perceive the affinity of Tanais through 
the respiratory apparatus with the larva of the Decapoda ; nay, 
more, it appears to us to range closer with the Amphipoda, for 
the pleopoda in Tanais closely resemble the same appendages 
in the Amphipoda, while the respiratory process is carried on 
by means of branchial sacs of similar form to those of the Am- 
phipoda, but of which a single pair only are attached to the 
third pair of pereiopoda. The circumstance of the respiratory 
process being carried on beneath the carapace occurs only in 
those animals in which, and only to the same extent as, the 
carapace covers the appendages of the pereion, and no more. 
Neither do we recognize the similitude of the cephalon in 
Tanais to the carapace of the Decapod Crustacea, as stated 
by Dr. Muller and Prof. Van Beneden. In the Decapods the 
carapace is indubitably the result of a monstrous growth of 
the mandibular somite, and its consequent overlying a greater 
or less number of the somites of the pereion; but in Tanais ^ 
as also in Apseudes, the reseinblance to a carapace is the result 
of a union between the cephalon and the first somite of the 
pereion. 
IV. Dr. Muller discusses the variation in the form of the 
second pair of gnathopoda as exemplified in two varieties of 
Tanais, as also in the new species, Orchestia darwinii, of which 
he gives a figure. He remarks that, when found near the shore, 
the form of the second pair of gnathopoda varies from that of 
specimens found at a distance inland, where it lives under 
mouldy leaves in loose earth which the marsh-crab {Gelasimus 
and Sesarmu) throws out at tlic entrance of its hole. He also 
notices the extremely large chelae of the Gelasimi, observing 
