282 
ZOOLOGICAL LITERATURE. 
caudal extremity^ which terminates on each side in one long and 
one short spine, the intermediate space being slightly concave. 
In the centre of the anterior margin is a small, black, clearly-de- 
fined eye. To the lower surface of the body are attached three 
pairs of appendages. The first is unibranched, furnished with 
a few simple hairs, and situated close to the frontal margin. 
The second is immediately behind the first ; it is hiramose, and 
has both branches furnished with ciliated hairs. These two pairs 
of appendages are nearly as long as the animal, hut the third 
pair is only half that length ; like the second, it is hiramose, 
and furnished with ciliated hairs on each branch. 
The little creature is rather opaque and of a brownish 
colour, darkest towards the extremities of the appendages. 
It is by these little appendages that the young animal swims, 
lashing the water in a manner that Dr. Muller compares to a 
man floating perpendicularly, with outstretched arms, having 
slender willow branches in each hand, working his way upward. 
The first progressive change observable is that of a larva 
slightly larger, being 0‘5 mill, in length. The colour and append- 
ages resemble those of the previous specimen ; but the posterior 
extremity of the body is drawn out into two thick conical pro- 
cesses, at the apices of which remain the two long caudal hairs, 
as previously observed, to which several others less prominent 
have been added. The number of hairs' upon the natatory 
appendages has increased also. 
At this stage the first indication of the carapace is seen in the 
presence of a transverse line across the dorsal suiTace of the 
animal. In this we see an important variation from the forms 
of either the Cirripedia or Decapod Crustacea, but one that 
assimilates it to the progressive changes of the larvae of certain 
Schizopoda. 
In the larvae of the Decapods and Cirripeds the carapace is 
defined from the very commencement. In that of Mysis 
and other Schizopoda the form of the animal is more perfectly 
annulose, and the growth of the anterior somites into a great 
dorsal protecting shield, is one of progressive development. In 
the larva of PeruBus the line which defines the future carapace 
is immediately behind the third pair of appendages — that is, 
behind that pair of appendages which ultimately become the 
mandibles, anterior to which, and just before the mouth, is a 
very large helmet-shaped protuberance, which Dr. F. Muller 
denominates the upper lip. In the anterior pair of appendages, 
the homologue of the future anterior pair of antennae, the de- 
velopment of some organ has evidently commenced ; while in 
the enlargement at the base of the posterior pair the outline of 
the future mandible is distinctly recognizable, showing that after 
the succeeding moult these organs will have lost, with their hair- 
like attachment, their pediform character, and pass into the 
