44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



segmental regions. The most anterior of these soon develops a bilobed 

 prominence which is the anlage of the labrum (pi. 3, fig. 34; pi. 10, 

 fig. 75 ; pi. 11, fig. 82) . It is known as the labral, ocular, or preantennal 

 segment. The second segment gives rise to the prominent paired 

 antennae and is therefore termed the antennal segment. The third 

 procephalic segment, which really arises in the region of transition 

 between the procephalic and protocormic regions, is much less promi- 

 nent. It does, however, produce a pair of small evanescent rounded 

 protuberances which may be regarded as homologs of the second 

 antennae of crustaceans. The protocormic region gives rise to 17 

 segments plus an additional incomplete one which is to be regarded 

 as the telson. The first three of these are called the gnathal segments 

 as they produce appendages which are differentiated into the larval 

 mouthparts (pi. 10, fig. 74). The most anterior is the mandibular 

 segment and, as its name indicates, its paired appendages form the 

 mandibles. Similarly the second or maxillary segment gives rise to 

 the maxillae, and the third or labial segment produces the labium. 

 This organ arises as a pair of appendages (the second maxillae) 

 whose bases soon fuse to form the bilobed labium of the larva. 



The three procephalic and the three protocormic segments described 

 above soon lose their lines of demarcation and become closely fused 

 to form the syncephalon of the developing larva. By contrast, all the 

 remaining protocromic segments, except the most caudal one, retain 

 their external identity even to the end of the larval period. The 

 three segments immediately posterior to the gnathal ones become 

 the prothoracic, the mesothoracic, and the metathoracic segments 

 respectively. 



The 11 segments of the protocormic region situated caudad to the 

 thoracic region constitute the abdomen. The eleventh, which is con- 

 siderably smaller than the rest, does not remain visible from the 

 outside for long as it is soon carried inward by the invagination of 

 the proctodaeum and becomes telescoped within the tenth segment. 



The general changes in body shape may be outlined at this point. 

 After 3 days of development, the germ band is so curved dorsally that 

 the anterior and posterior ends nearly touch (pi. 10, figs. 73, 74, 75). 

 During the fourth day the embryo shortens so that it reaches only to 

 the poles of the egg. The two extremities of the embryo are still 

 turned dorsally, however, and they remain so flexed until early in the 

 fifth day, when the embryo straightens out and then curves ventrally 

 (pi. 12, fig. 87). Coinciding with this ventral flexure, there is gradu- 

 ally an elongation of the embryo until it is so long that the curvatures 

 of the ends are insufficient to allow its accommodation within the egg. 



