266 CRUSTACEA. 



Acanthosoma stages are noteworthy on account of the development of the 

 auditory vesicle at the base of the first antenna and the further growth of 

 the rudiments of the pleopoda. 



The Mastigopus form which develops from the Acanthosoma is very like 

 that of Lucifer. Here also we find great developmont of the abdomen as 

 opposed to relatively small size of the cephalo-thorax. The armature of spines, 

 with the exception of the persistent rostrum, is reduced to small vestiges. 

 The exopodites are lost on the thoracic limbs (maxillipedes and ambulatory 

 limbs), and the last pair of these limbs have altogether disappeared. The 

 pleopoda are now greatly developed, but still have no exopodites ; these, 

 however, are evident as buds on the two posterior pairs, but only attain 

 functional development at a later stage. 



The changes by which the Mastigopus passes into the adult form chiefly affect 

 the limbs (above all the mouth-parts), which now approach the adult form ; 

 the mandibular palp, for instance, grows out, the two last pairs of thoracic 

 limbs reappear, and the gills develop. The developing mandibular palp remains 

 two-jointed in Sergestes; the anterior maxilla in the later Mastigopus stages 

 still shows a short rudiment of the palp. In the posterior maxilla, on the 

 contrary, the exopodite is transformed into a large, fan-like plate. In the 

 anterior maxillipede, the exopodite and endopodite are short appendages, while 

 a large, plate-like, masticatory blade has developed on the protopodite. The 

 second maxillipede is short and geniculate, while the third has retained the form 

 of a long limb. Even in early stages, the rudiments of the pincers are recog- 

 nisable on the second and third ambulatory limbs. The loss of the Nauplius 

 eye and the shell-gland, and the development, on the other hand, of the 

 antennal gland are noticeable features in the internal anatomy. 



In the absence of gills and of the two posterior pairs of thoracic limbs, 

 Lucifer shows features which are present in Sergestes at the Mastigopus stage. 

 Lucifer has thus retained certain larval characteristics. 



The many larval stages of Sergestes found among the Challenger material 

 agree in the most essential points with the stages above described, but show 

 great variability in the armature of the cephalo-thorax and abdomen. The 

 Sergestid Zoaea called Platysaccus crenatus is interesting ; its dorsal shield, 

 which is rounded and provided with marginal spinous lobes, leaves the four 

 posterior thoracic segments (already provided with limbs) completely uncovered, 

 a feature in which this form agrees with the Penaeus and Lucifer larvae. Some 

 larvae of the Mi/sis and Mastigopus stage called Sciocaris telsonis are remarkable 

 for the shape of the telson, which has segmented furcal processes. 



The contrasts in outward appearance presented by the short, broad, and 

 spiniferous larva of Sergestes and the slender larval form of Lucifer are brought 

 about by a series of ontogenetic stages, which Brooks (No. 109) has traced back 

 to a meeting point in the genus Acctcs. These larvae are more compact than the 

 Lucifer larva, and are also somewhat better supplied with spines. Brooks is 

 inclined to refer to this series a Protozoaea larva described by Dohrn (No. 121, 

 "Larva of an unknown Crustacean," Plates 29 and 30, Figs. 62-67) and Ci .vis 

 (No. 8, " Phyllopoda-like Protozoaea of unknown ancestry," Plate 4, Figs. 2-7). 

 The latter larva is principally characterised by the great development of the still 

 unstalked lateral eyes, which, as in Lucifer, cause a buckle-shaped projection 

 of t lie dorsal carapace. With respect to the condition of the stalked eye, 

 which is already wry highly developed in the Protozoaea of Sergestes, this larva 

 holds a position intermediate between the latter and Lucifer. 



