288 



ZOOLOGY. 



pole of the egg. This single cell subdivides, its products 

 forming the " blastodermic disk" or outer gerin-layer, the 

 segmentation of the yolk being partial. The 

 third (innermost) and middle germ-layers next 

 arise (the same processes go on in certain 

 shrimps, viz. : Crangon and PdlcBmon). 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 antennae in Oniscus 

 and also Asellus are the first to bud 

 out ; the remaining appendages of 

 the head and thorax arise contem- 

 poraneously, and subsequently the 

 abdominal feet. The abdomen in 

 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 (G-ammarus) partially segment, as 

 in the spiders, and in a less degree the insects. 



Standing next below Cymothoa, which is of the general 

 Isopod 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 a representative. The females (Fig. 257) are par- 

 asitic under the carapace of various shrimps. In B. palcemon- 

 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 



palpus. Drawn 

 by J. S. Kings- 

 ley. 



Fig. 254. - 

 gill of Serolis. 

 Drawn by J. S. 

 Kingsley. 



