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 tlie pleopoda. 



The Mastlgopus form which develops from the Acanthosoma is very like 

 that of Lucifer. Here also Ave find great developmont of the ahdomen as 

 opposed to relatively small t-ize of the cej)halo-thorax. The armature of spines, 

 with the exce])tiou 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 pa.sses into the adult form chiefly att'ect 

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

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

 limbs rea])pear, and the gills develop. Tlie developing mandibular palp remains 

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

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

 contrary, the exo])odite is transformed into a large, fan-like plate. In the 

 anterior maxillipede, the cxopodite and endopodite are short ajtpendages, wbilc 

 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 Nai;plius 

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

 anteunal 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 Sergcstcs at the Mastigopus stage. 

 Liicifer has thus retained certain larval characteristics. 



The many larval stages of Scrgestcs 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 ajipearance presented by the short, broad, and 

 spiniferous larva of Sergestes and the slender larval fi)rm of Lucifer are brouglit 

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

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

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

 inclined to refer to this series a Frotozoaea larva described by DoHUX (No. 121, 

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

 (No. 8, " Phyllopoda-like I'rotozoaea 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 the dorsal carai)ace. AVith respect to the condition of the stalked eye, 

 which is already very highly developed in the Protozoaca of Sergcstcs, this larva 

 holds a position intermediate between the latter and Lucifer. 



