288 ZOOLOGY. 
pole of the egg. This single cell subdivides, its products 
forming the “‘blastodermic disk” or outer germ-layer, the 
segmentation of the yolk being partial. The 
. third (innermost) and middle germ-layers next. 
\ arise (the same processes go on in certain 
RSs shrimps, viz.: Crangon and Palemon). The 
intestine is formed by an in-pushing of the 
outer germ-layer. The limbs now bud out, the 
result of the pushing out of the outer germ- 
layer (ectoderm). The nervous cord arises from. 
=~ the ectoderm ; the large intestine originates in 
> the yolk-sac, its epithelium first 
appearing in the liver-sac. The 
heart is the last to be formed. Ex- 
ternally the antenne in Oniscus 
Fie. 253,— andalso Asellus are the first to bud 
Mouth-part of cut; the remaining appendages of § 
dible jee sree the head and thorax arise contem- @ 
maxilla; ma”, Fig. 254.-—A. 
second maxilla; poraneously, and subsequent] ‘ 4. - 
mp, maxillary P couse b eq t y the ill of Serolis.— 
palpue.—Drawn abdominal feet. The abdomen in xinnjey! 7 * 
ky.» the Isopods is curved upwards and 
backwards, while in the embryo Amphipods it is bent be- 
neath the body. 
The development of the Amphipods or beach-fleas is. 
nearly identical with that of the Isopods. The eggs of cer- 
tain species undergo total segmentation, while those of other 
species of the same genus (Gammarus) partially segment, as. 
in the spiders, and in a less degree the insects, 
Standing next below Cymothoa, which is of the general 
Tsopod shape, but which lives parasitically on the tongue: 
and other parts of fishes, but which from their parasitic 
habits become slightly changed in form, the females espe- 
cially, sometimes becoming blind, is the family of which. 
Bopyrus is arepresentative. The females (Fig. 257) are par- 
asitic under the carapace of various shrimps. In B. palemon- 
eticola Packard, the females are many times larger than 
the males; the ventral side of the body is partly aborted, 
having been absorbed by its pressure against the carapace 
of its host, which is swollen over it ; it retains its position by 
