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 

 shrimps, viz. : Crangon and Pdlcemon). 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 

 Fi - 253 _ and also Asdhis are the first to bud 

 Monti-parts of ou . t he remaining appendages of 



Serolu. w.raan- - 1 * 



dibie ; mx', first the head and thorax arise contem- 



muxilla ; in .'", , ., ,, pi c 354 _A. 



second maxilla; poraneously, and subsequently the gi n j seroiis. 



mf), muxillary n-.-. T>i*au'n hv T S 



paipus.-Drawn abdominal feet. The abdomen m KingViey. 

 iey. J ' 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, Avhich is of the general 

 Isopod shape, but Avhich 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 Avhich 

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

 asitic under the earn pace of various shrimps. In B.palcBmon- 

 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 



