CRUSTACEA. 391 



ance 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 maxillie are provided with small respiratory plates 

 (exopouites) for regulating the flow of water under the dorsal shield. 

 From the Zosea 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 endo- 

 podites become functional. At the same time the antennae become 

 profoundly modified. The anterior antennae shed their long hairs, and 

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

 which eventually elongates and becomes the inner flagellum. The 

 outer ramus of the posterior antennae is reduced to a scale, while the 

 flagellum is developed from a stump-like rudiment of the inner ramus 

 (Glaus). A palp sprouts on the mandible and the median eye dis- 

 appears. 



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 



FIG. 216. PEN^US LARVA IN THE MYSIS STAGE. (After Clans.) 



ambulatory function, cutting plates develop on the inner side of their 

 basal joints, and the two rami persist 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 develop- 

 ment and that on the first maxilla disappears 1 . The Nauplius, so far 

 as is known, does not occur in any other Decapod form except 

 Penseus. 



1 From Clans' 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 embryologies! elucidation. 



