ON DEVELOPMENT OP ATYEPHIEA COMPEESSA. 
423 
The extremity of the first pair of antennae (fig. 80, At. 1, and 
fig. 95). is at this time furnished with four setae. At the base 
of each of the two inner setae there is a nucleus enclosing a 
darkly-staining nucleolus. No definite external distinction 
can as yet be made between these and other setae. Gradually, 
however, these become distinct from others (fig. 96). Granu- 
lations appear on the upper part of the two setae destined to 
become the olfactory organ. A constriction occurs at the 
base of these setae, where the nerve-fibre, running from the 
ganglionic mass, ends in an enlargement. At the time the 
embryo is hatched they show the appearance shown in fig. 96. 
After some time, when the embryo is about 4 mm. in length, 
they become spatula-shaped, their ends present the form of a 
prominent papilla (fig. 97). 
At each moulting they appear in twos on the distal end of 
each segment, in the same manner as the two first were 
formed. 
The further changes of the embryo correspond in the main 
with what has been observed by Faxon on Palaemonites 
( 3 ,°. 3 ,- ? 32 . 3 ). After each moulting the embryo gets more of the 
characters of the adult. The last pair of abdominal appen- 
dages become visible as two oval plates within the caudal fin 
soon after the first moulting. The other appendages are regu- 
larly formed from behind forward, while the number of the 
ambulatory legs becomes complete. 
In the embryo measuring 4 mm. (i. e. two moults after the 
hatching of the embryo) the caudal fin or telson separates from 
the sixth abdominal segment. 
Branchiae other than those already mentioned are regularly 
formed. A rudiment of the podobranchiae is first seen at the 
coxopodite of the second pair of ambulatory legs as a simple 
sac, similar to those of the pleurobranchiae. These (podo- 
branchiae) next appear almost simultaneously on the other am- 
bulatory legs excepting the last, and then successively on the 
third, second, and first maxillipedes. Of these only one pair 
(i. e. that on the second pair of maxillipedes) develops into 
a permanent gill, while five others (those on the third pair of 
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