101 



embryo in the form of a double series of bud-like prominences. Of these, howevef, 

 the 3 anterior pairs are somewhat in advance of the others in development, being 

 rather distinctly defined, while the other 9 pairs can be but faintly traced (see 

 tig. 4). These 3 anterior pairs of limits represent the 2 pairs of antennae and 

 the mandibles, which, as is well kno\\n, are also those first appearing in other 

 Crustacea, characterising the so-called Nauplian stage. The 1st pair (a 1 ), repre- 

 senting the superior antennae, are much larger than the other 2, and have the 

 form of 2 spatulate plates, slightly bilobular at the tip, and extending backwards 

 on the sides of the embryo. The 2 succeeding pairs (inferior antennae and 

 mandibles, a-. M) also extend somewhat laterally, and are simple conic in form. 

 Between them in the middle, the ventral face projects as a rounded prominence 

 (L), representing the anterior lip or labrum. The remaining 9 pairs of limbs, as 

 stated above, are at first but very faintly defined and of rather uniform appear- 

 ance, causing a slight crenulation of the ventral face. 



In a succeeding stage (fig. 5) the egg-membrane or chorion is cast off, 

 and the 3 anterior pairs of limbs thereby become freely projecting, whereas the 

 others are still enveloped by the thin larval skin. The dorsal cleft has deepened 

 considerably, and extends somewhat obliquely behind, whereby the anterior 

 broadly rounded extremity is defined clearly from the somewhat narrower 

 posterior extremity. The latter has given origin on each side to a knoblike 

 prominence (urp), indicating the developing uropoda. The 3 anterior pairs of 

 limbs have not greatly changed their appearance. The remaining pairs, on the 

 other hand, now appear very distinctly, and the 2 anterior of these (m 1 , m 2 ), 

 representing the 2 pairs of maxillae, distinguish themselves from the succeeding 

 7 pairs both by their form and more lateral position. They are also placed 

 somewhat more apart, though the anterior pair partly cover the posterior. The 

 former has developed a small lateral ramus (the reflexed palp), and the latter 

 appears slightly trilobate at the end. The 7 succeeding pairs of limbs have 

 a very similar appearance, each terminating in a conical, posteriorly-curving 

 process (the endopodite), and having outside a knob-like protuberance (the 

 exopodite), which, however, in the 2nd and 2 last pairs is almost obsolete. In 

 the 3 middle pairs, representing the 3rd pair of maxillipeds (mp 3 ) and the 2 

 first pairs of legs (p 1 , p 2 ) this protuberance is easily observable; and in the 

 1st pair (mp 1 ), representing the 1st pair of maxillipeds, an evidently homologous 

 outer appendage is also seen; but it here issues somewhat higher up than in the 

 other pairs, and subsequently also becomes different in form and direction. It 

 represents the exopodal part of the branchial apparatus, the epipodal part being 

 apparently not yet formed. Just above the maxillae, there is seen on each side of 



U Crustacea. 



