472 DECAPODA. 



their swimming feet. The early appearance of the appendages 

 of the sixth abdominal segment is probably correlated with 

 their natatory function in connection with the tail. As a point 

 of smaller importance which may be mentioned is the fact that 

 both pairs of maxillae are provided with small respiratory plates 

 (exopodites) for regulating the flow of water under the dorsal 

 shield. From the Zoasa form the larva passes into a Mysis or 

 Schizopod stage (fig. 216), characterised by the thoracic feet and 

 maxillipeds resembling in form and function the biramous feet 

 of Mysis, the outer ramus being at first in many cases much 

 larger than the inner. The gill pouches appear at the base of 

 these feet nearly at the same time as the endopodites become 

 functional. At the same time the antennas become profoundly 

 modified. The anterior antennas shed their long hairs, and from 

 the inner side of the fourth joint there springs a new process, 



FIG. 216. PEN^EUS LARVA IN THE MYSIS STAGE. (After Claus.) 



which eventually elongates and becomes the inner flagellum. 

 The outer ramus of the posterior antennas is reduced to a scale, 

 while the flagellum is developed from a stump-like rudiment of 

 the inner ramus (Claus). A palp sprouts on the mandible and 

 the median eye disappears. 



The abdominal feet do not appear till the commencement of 

 the Mysis stage, and hardly become functional till its close. 



From the Mysis stage the larva passes quite simply into the 

 adult form. The outer ramus of the thoracic feet is more or less 

 completely lost. The maxillipeds, or the two anterior pairs at 

 any rate, lose their ambulatory function, cutting plates develop 

 on the inner side of their basal joints, and the two rami persist 



