CRUSTACEA. 



473 



as small appendages on their outer side. Gill pouches also 

 sprout from their outer side. 



The respiratory plate of the second maxilla attains its full 

 development and that on the first maxilla disappears 1 . The 

 Nauplius, so far as is known, does not occur in any other 

 Decapod form except Penaeus. 



The next most primitive 

 larval history known is 

 that which appears in the 

 Sergestidae. The larval 

 history, which has been 

 fully elucidated by Claus, 

 commences with a Proto- 

 zoaea form (fig. 217), which 

 develops into a remarkable 

 Zoaea first described by 

 Dohrn as Elaphocaris. 

 This develops into a form 

 originally described by 

 Claus as Acanthosoma, 

 and this into a form known 

 as Mastigopus (fig. 218) 

 from which it is easy to 

 pass to the adult. 



The remarkable Proto- 

 zoasa (fig. 217) is charac- 

 terised by the presence on 

 the dorsal shield of a fron- 

 tal, dorsal and two lateral 

 spikes, each richly armed 

 with long side spines. The 



FIG. 217. LATEST PROTOZO/EA STAGE OF SEK- 



GESTES LARVA (ELAPHOCARIS). (After ClaUS.) 



Hixp'" ' . third pair of maxillipeds. 



normal Zoaea appendages are present, and in addition to them a small third 

 pair of maxillipeds. The thoracic region is divided into five short rings, but 

 the abdomen is unsegmented. The tail is forked and provided with long 

 spines. The antenna;, like those of Penaeus, are long the second pair 

 biramous ; the mandibles unpalped. Both pairs of maxillae are provided 

 with respiratory plates ; the second pair is footlike, and has at its base a 

 glandular mass believed by Claus to be the equivalent of the Entomostracan 

 shell-gland. The maxillipeds have the usual biramous characters. A 



1 From Claus' observations (No. 448) it would appear that the respiratory plate 

 is only the exopodite and not, as is usually stated, the coalesced exopodite and 

 epipodite. Huxley in his Comparative Anatomy reserves this point for embryological 

 elucidation. 



