152 CRUSTACEA. 



the last (sixth) pair being still wanting when the young animal is hatched 

 (Fr. Muller, No. 4 ; Claus, No. 78). It has already been shown (p. 151) that 

 the paired disc-like thickenings observed by Fr. Muller in the Tanais embryo 

 are probably to be referred to the dorsal organ. 



The embryos of the Cumacea also, through the absence of the seventh pair of 

 thoracic limbs, and the presence of a dorsal organ (resembling that of Cymothoa) 

 are allied to the Isopoda. As in the Anisopoda, the embryo when hatched 

 possesses only one (the sixth) pair of abdominal limbs. 



C. Leptostraca, Schizopoda, Decapoda. 



There are two factors which, as a rule., influence the ontogenetic 

 development of these groups. (1) The food-yolk which, in most 

 cases, is present in very considerable quantities, and which determines 

 the size of the egg and the entirely superficial extension of the 

 embryo in the first stages; (2) the gradual development of the long 

 germ-band out of an originally short rudiment consisting of a few 

 segments (distinctly marking the Nauplius stage). 



Nothing further is known of the presumably very primitive condition shown 

 in the development of the larval body of those genera which leave the egg in the 

 Nauplius form (Eaphausia, Penaeus, Lucifer). 



We may take the ontogeny of Mysis as the starting-point of our 

 description, following chiefly E. Van Beneden (No. 37) and 

 Nusbaum (No. 39). The eggs of Mysis develop (as in the Cumacea 

 and Arthrostraca) in a brood-sac covered by horizontally- placed 

 lamellae (oostegites) of the thoracic limbs. The first development 

 of the embryo appears at the point from which the formation of the 

 blastoderm originated, and which corresponds to the future ventral 

 surface. The formation of the blastoderm is commenced by the 

 development of a rounded disc, the germ-integument, which ultimately 

 spreads over the surface of the egg; a later stage shows a similar 

 rounded blastodermic thickening (the germ-disc) which represents the 

 first embryonic rudiment. This soon breaks up into three lobes, 

 the middle one representing the rudiment of the caudal region, and 

 the paired lateral lobes the rudiments of the germ -band. The latter 

 soon grow forward as two diverging bands, from which the rudi- 

 ments of the Nauplius limbs bud out as rounded prominences 

 (Fig. 77 A). The broadened anterior ends (o) of these halves of the 

 germ-band correspond to the cephalic lobes of the Entomostracan ami 

 Arthrostracan germ-bands. As they give rise exclusively to the 

 rudiments of the compound eyes and the optic ganglia, we shall 

 give them the more precise name of optic lobes. On either side 

 of the short germ-band, between but somewhat above the first and 

 second pairs of antennae, lies the disc-shaped paired rudiment of the 



