284 
ZOOLOGICAL LITERATURE. 
at the base of this newly-formed rostrum, the solitary eye of the 
embryonic larva is still visible. 
The antennae continue to be the chief instruments of locomo- 
tion, in which these animals offer an essential difference from the 
larvae of the Brachyura, Anomura, and many of the Macrura 
and Stomapoda, where locomotion is produced by the aid of the 
gnathopoda. 
This fact appears to he in accordance with a uniform laAv in 
the development of these animals. When the larva quits the 
ovum in an early stage, the antennae become the organs of loco- 
motion. If in a later stage, then the appendages of the mouth 
are the organs by which locomotion is produced; but those 
which are retained in an embryonie condition until a still later 
stage of development are furnished with those organs of loco- 
motion which they retain through life. 
In stating that the organs of loeomotion in the larvai of 
Pe7iceus, Cirripedia, and others, are the antennae, it must be re- 
membered that they are not the antennae of tlie adult animal, 
but deciduous appendages that are wholly replaced by permanent 
ones; Avhereas in the larvae of the Brachyura the deciduous 
organs represent, not the entire adult organ, but only the 
secondary parts of the saime, the permanent appendages budding 
out from the base of the deciduous organs, which are afterwards 
lost. In the third stage, represented by the fresliwater and ter- 
restrial Crustacea, no deciduous appendages exist at any period, 
but the embryo is retained under incubation until the permanent 
appendages are formed. 
Dr. Miille^" traces the progressive development of the several 
organs, of which the pair of permanent eyes are the first which 
make their appearance. 
The new somites, to which the appendages of the pereion and 
pleon are attached, form at first an nnjointed, soft, short band, 
in which very rapidly the demarcation of eleven somites may be 
detected before it has attained the length of that portion of the 
pleon which exists posteriorly to it. This is shortly followed by 
the development of their respective appendages, which at first 
are but small foliaceous lobes; these afterwards become biramous 
appendages, and give to the animal a close affinity to that of a 
young stomapod. It is this stage that Dr. Muller calls the ^‘^Mysis 
form.^^ In this form the larva was observed before it was 2 mill, 
in length, and it continued to grow in dimensions until it was 
4’5 mill, in length. It was during this period that the auditory 
organs, the chelate and other j)ereiopoda were developed, as well 
as the branchial organs, the mandibular appendages, and ple- 
opoda. 
Dr. Muller likewise describes the progressive a[)pearaiice of 
the auditory a))paratus. IVom this stages there is but little 
alteration to that of the young prawn. The frontal [)roeess has 
