120 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The Appendages. 
The order of the appearance of the appendages of Penilia departs 
from the usual rule for Crustacea. The first or sensory antennae 
do not appear first but only after the second antennae are clearly 
distinguishable, and in some cases where the retardation is excep¬ 
tional, they may not appear sharply differentiated until the second 
antennae, mandibles, and maxillary region are plainly formed. A 
maxillary region is differentiated before the thoracic appendages 
appear and the appendages themselves do not develop until rudi¬ 
ments of every other appendage of the entire animal are developed. 
The thoracic appendages appear in the usual order, as the most 
anterior is the oldest and those following show a perfect gradation 
in their develojmient. The appendages will be taken up in the 
order of their position and not in reference to the time of their 
appearance. 
First Antennae. 
The first or sensory antennae, as has been just said, vary consider¬ 
ably as to the time of their appearance. They begin as angles on 
either side of the rounded anterior end of the larvae. They may 
appear in this way when the second antennae are just beginning to 
be plainly visible or they may not appear until the mandibles and 
maxillary region are marked off. This angle increases in prominence 
until it gradually forms a rounded lump projecting anteriorly, its 
connection with the body of the embryo being the smallest part 
of its diameter. (Fig. 28.) Its relation to the entire animal is 
shifted as the bending of the head and the outgrowth of the upper 
lip occur. From its extreme anterior position in front of the second 
antennae it changes to the ventral side of the embryo, and finally in 
the older embryos and adult its position is ventral and posterior to 
that occupied by the second antennae. The first antennae project 
on either side from outgrowths or horn-like processes developed 
from a part of the upper lip. They remain small throughout life 
and in the adult consist of but a single joint with nerve-cells and 
sensory fibers or setae. 
