392 



DECAPOD A. 



The next most primitive 

 larval history known is that 

 which appears in the Ser- 

 gestidse. The larval history, 

 which has been fully eluci- 

 dated by Claus, commences 

 with a Protozosea form (fig. 

 217), which develops into a 

 remarkable Zosea first de- 

 scribed by Dohrn as Ela- 

 phocaris. 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- 

 zosea (fig. 217) is charac- 

 terised by the presence on 

 the dorsal shield of a frontal, 

 dorsal and two lateral spikes, 

 each richly armed with long 

 side spines. The normal 

 Zoaea appendages are pre- 

 sent, and in addition to them 

 a small third pair of max- 



illipeds. 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 PenaBus, are long the second pair 

 biramous ; the mandibles unpalpecL 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 Eutomostracan 

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

 helmet-shaped upper lip like that of a typical Nauplius is present, and the 

 eyes are situated on very long stalks. 



In the true Zorea stage there appear on the five thoracic segments 

 pouch-like biramous rudiments of the limbs. The tail becomes segmented ; 

 but the segments, with the exception of the sixth, remain without 

 appendages. On the sixth a very long bilobed pouch appears as the 

 commencement of the swimming feet of this segment. The segments of 

 the abdomen are armed with lateral spines. 



From the Zosea stage the larva passes into the form known as Acan- 

 thosoma, which represents the Mysis stage of Penanis. The complex spikes 

 on the dorsal shield of the Zosea stage are reduced to simple spines, but 

 the spines of the tail still retain their full size. In the appendages the 

 chief changes consist (1) in the reduction of the jointed outer ramus of the 

 second pair of antennae to a stump representing the scale, and the elon- 

 gation of the inner one to the flagellum ; (2) in the elongation of the five 

 ambulatory thoracic appendages into biramous feet, like the maxillipeds, 

 and in the sprouting forth of rudimentary abdominal feet. 



FIG. 217. LATEST PKOTOZO^EA STAGE OF SEE- 



GESTES LARVA (ELAPHOCARIS). (After ClaUS.)' 



mxp"'. third pair of maxillipeds. 



