308 



ZOOLOGY. 



pole of 

 forming the 



the eg'Sf. 



Fig. 2 5 8.— 

 Mouth-parts of 

 Serolu. m, man- 

 dible ; vix', first 

 maxilla ; 7nx'\ 

 second maxilla ; 

 inp, mjixillary 

 palpus.— Drawn 

 by J, S- Kings- 

 ley. 



This single cell subdivides, its products 

 "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 Palwmon). 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 (Gammarus) 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 iishes, but which from their parasitic 

 habits become slightly changed in form, the females esjje- 

 cially, sometimes becoming blind, is the family of which 

 Bopyrus is a representative. The females (Fig. 262) are par- 

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

 icola Packard n. s]d., 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 b^ 



Fig. 259. — A 

 gill of SerolU. — 

 Drawn by J. S. 

 Kiugsley. 



