INTRODUCTION. xh 
generally fringed with hairs. One of these plates is 
developed on the inner side of each of the two pairs of 
gnathopoda (Fig. 17), and the two 
an terior pairs of pereiopoda, These 
plates overlie each other in a compact 
form, securely protecting the ova, or 
the immature young, from external 
accidents, as shown in fig. 10, p. xxxiil. 
It is the opinion of Von Siebold 
that these appendages are periodically 

developed at the “époque du rut.” 
Fig. 17. This we have not, from our own obser- 
vation, been able to verify, having taken females during 
all periods of the year with these appendages fully de- 
veloped. They are absent on the young females. We 
believe, however, that, when they are once developed, 
they continue permanent organs, only disappearing as the 
result of accident. 
In the Anceide, the incubatory pouch appears to belong 
to the three posterior segments of the pereion. By the 
continued growth of the ova, the pereion is reduced to a 
most impoverished state. The alimentary canal being in 
a collapsed condition, and always empty, the animal can 
only be viewed in the light of a great egg-producer, after 
the development of which an empty sac only is left, the 
poor remains of a worn-out animal. 
The history of the development of the ovum from 
its impregnation to the development of the perfect larva 
has been best worked out by Valette St. George in 
the Amphipoda, and Anton Dohrn in the Jsopoda. We 
must refer the student to the memoirs of these two 
authors for a detailed account of the germination and 
growth of the ovum in all its stages. It will suffice for 
us to say, that it appears to be clearly established by 
