CR USTACEA— ONTOGENY 



397 



traca (Xt'balia), the AmpMpoda, and a few Decapoda {e.g. the Cray-fish), leave 

 the egg in a form like that of the sexually ripe animal. On the other hand in 

 the Isopoda, Mysidce, and Lophogastridai among the ScMzopoda and the Cumacea, the 

 young form hatched from the egg may be very little developed, and may even 

 resemble a maggot-shaped Nauplius or MetanaupUus ; but there are no early free- 

 swimming larval forms, as the young undergo their metamorphoses in the brood 

 pouch of the mother. The development of the parasitic Isopoda is interesting, since 

 here the free-moving young or larval forms which go in search of hosts, resemble the 

 typical Isopoda in form and development of the body and limbs much more distinctly 

 than do the sexually mature animals. 



5 B 



Fig. 269.— Moina reotiroatrls. Four early stages of development. A, Blastosphere, seen 

 from the vegetative pole. B, Gastnila stage. C, somewliat older stage, with neural plate, closed 

 blastopore and sexual cells sunk below the surface ; D, Nauplius stage, B, C, D, in optical 

 longitudinal section. In C, D, the neural plate, and in D, the rudiments of the Ncmplius limbs are 

 projected on to the section, cc, Ectoderm ; me, mesoderm ; g, primitive sexual ceUs ; en, endodenn ; 

 sj, neural plate ; ftp, blastopore ; nd, nutritive yolk ; ro, mouth ; st, stomodajum ; ay anterior ; a-<, 

 pasterior antennae ; md, rudiments of mandible (after Grobben). 



B. The Arrangements of the Germ Layers and the Development of the 

 Inner Organs 



may here be described by means of a few examples. 



I. Moina reotirostris (Order, Phyllopoda ; Sub-order, Cladocera ; Fig. 269). 



The segmentation of the mesolecithal egg is superficial, and somewhat unequal. 

 In the 32 blastomere stage a blastomere lying at the vegetative pole is distinguished 



