SERGESTIDAE. 263 



guished by the segmentation of the second antenna and the great 

 development of the abdomen, show, on the five anterior abdominal 

 segments, the bud-like rudiments of the pleopoda. 



The transition from the Mysis stages to the adult form takes 

 place through the Mastigopus stage (Fig. 120 C), which in length 

 of body already approaches the adult Lucifer, but is distinguished 

 from the latter by the absence of the neck-like prolongation of the 

 cephalo-thorax. This stage is marked by the shortness of the 

 flagellum of the first antenna (a'), while the flagellate appendage 

 of the second antenna {en) has considerably lengthened. The mouth- 

 parts and the thoracic limbs have attained their final condition. The 

 mandible has no palp, the first maxilla has lost the exopodite, 

 the latter portion of the second maxilla is transformed into a large, 

 fan-like plate. The first maxillipede is changed into a short, two- 

 jointed appendage ; the second maxillipede {mf") has, like all the 

 other thoracic limbs, lost its exopodite and has lengthened out and 

 become geniculate. The third maxillipede and the three anterior 

 pairs of ambulatory limbs form a row of short, simple appendages 

 covered with setae. The fourth pair of ambulatory limbs has 

 entirely disappeared. The first pleopod (ab-^ is uniramose, while 

 the four following pleopoda {2-5) have the usual biramose form. 

 The Nauplius eye and the shell-gland have now disappeared ; on the 

 other hand, the antennal gland {dr), which opens on the base of 

 the second antenna, can be recognised as a coiled canal. 



The adult form, as contrasted with the Mastigopus stage, is char- 

 acterised by the lengthening of the flagellum of the first antenna, in 

 the basal joint of which the auditory organ develops, as well as by 

 the neck-like prolongation of the head. The flagellum on the second 

 antenna has also undergone considerable elongation. The sexual 

 differentiation now develops, the male being distinguished by 

 accessory structures on the first and second pairs of pleopoda, 

 by spines on the ventral side of the fifth abdominal segment, as 

 well as by certain differences in the caudal fin, while the female 

 has in these respects retained the characters of the larval form. 



The development of Lucifer just described is, in all essential points, repeated 

 in the metamorphosis of Sergestes, although the two larvae are somewhat unlike 

 in outward appearance. The Zoaea of Sergestes, which is remarkable for the 

 dorso- ventral flattening of its body and its extraordinarily large, branched, 

 spinous processes, was described by Dana under the name of Elaphocaris, the 

 later stages already being known as Acanthosoma (Claus, No. 91) and Mastigopus 

 (Leuckaut, Claus, No. 91). Claus and Willemoes-Suhm simultaneously 

 proved that these forms belonged to the ontogeny of Sergestes, and discovered 



