146 CRUSTACEA. 



indications of an arrangement into a paired band and a segmentation 

 of such a band can be found. Here again we are reminded of Ligia 

 and Cymothoa. Only a few slight observations are recorded as to 

 the appearance of metameric coelom-sacs ; these have already been 

 alluded to (pp. 122, 133, and 140). As a rule, the body-cavity of 

 the Crustacea develops like a pseudocode as a system of irregular 

 lacunar spaces within the mesoderm. The more these spaces extend, 

 the greater must be the interval between the surface of the body 

 and the mass of food-yolk enclosed by it. The cavity of the coelom 

 is, as a rule, filled with fluid ; though it should be mentioned that 

 Nusbaum found masses of food-yolk filling the spaces in the anterior 

 part of the embryo of Mysis. 



4. Development of the External Form of the Body. 

 A. Entomostraca. 



In Entomostraca whose eggs are poor in yolk, the embryos often 

 hatch at an early ontogenetic stage (as Nauplii) ; here the form 

 of the larval (Nauplius) body evolves very gradually from the 

 spherical shape of the egg. As the egg lengthens, the boundaries 

 of the separate segments of the Nauplius body appear as transverse 

 constrictions, while the limbs arise as outgrowths of the surface of 

 the body (Fig. 72 A), in which both the ectoderm and the subjacent 

 cell-mass of the mesoderm take part. These processes are to be 

 observed in the embryos of Branchipus, free-living Copepoda and 

 Cirripedia, and also in certain Cladoceran embryos distinguished by 

 the paucity of food-yolk (Moina, Grobben, Fig. 72). In the embryos 

 of those forms which pass the Nauplius stage within the egg, when 

 the posterior body-segments develop and the embryo thus lengthen?, 

 a dorsal curvature of these segments takes place (Apus productive, 

 Brauer; Moina, Fig. 72 B and C, Grobben). In these cases, the 

 rudiment of the cephalic carapace or shell (a) can be made out early 

 as a folding of the dorsal integument at a point corresponding to 

 the maxillary region. 



The development of the embryo in the yolk-laden eggs of many 

 Cladocera (e.g., Daphnia longispina, Dohrn, No. 10, and Leptodora, 

 P. E. Muller, No. 12) pursues a different course. In these 

 embryos, a certain contrast between the embryonic rudiment which 

 lies primitively on the ventral side of the egg and the dorsally- 

 placed mass of food-yolk is recognisable. This distinction is still 

 more evident in the embryos of parasitic Copepoda (Fig. 73 A and Ii 



