392 



COMPARATIVE ANATOMY 



CHAP. 



posterior pairs of thoracic feet (ambulatory feet) project freely as doubly- tipped pro- 

 tuberances. The formations on the abdomen of the 1st Zocea stage, which were 

 indicated as the rudiments of pleopoda, are no longer to be seen. The cephalothoracic 

 shield has a pointed process projecting anteriorly in the middle line. The animal still 

 moves chiefly by means of the 2 pairs of antennse. 



7. Mysis or Schizopoda Stage (Fig. 264, D). This is so called because all the 

 thoracic feet are developed, like those of the Schizopoda, as long biramose limbs 

 (with exo- and endopodite), and here also serve for swimming. The branchial appen- 



FIG. 264. Older larvae of Penaeus. A , Older Protozoaea, dorsal view. B, 6 posterior thoracic 

 segments, and abdomen, with the rudiments of the feet of a somewhat older larva. C, Further 

 advanced Zoaea. D, Mysis stage of a Penaeus, from the side, fs, Frontal sensory organ ; L, liver ; 

 ab, abdomen ; IV-VIII, thoracic segments ; (c^-og), abdominal segments ; t, telson. In C to the 

 left, the 3d thoracic foot (3d maxillipede) is covered by the 2d (after Glaus). 



dages of the thoracic feet appear. The pleopoda grow further during this period, the 

 most anterior pair first, then the 2d and then the others almost simultaneously. 

 The feelers undergo important transformations, which bring them nearer the adult 

 form. Alterations likewise appear in the cephalothoracic shield. The auditory sac 

 forms at the base of the antennae. The mandible receives a feeler. The jaws 

 approach their definitive form. By degrees, through several moults, the larvae reach 



8. The Penaeus form, the exopodites of the thoracic feet becoming more or less 

 reduced and the pleopoda developing further. 



The Penaeus development also shows us that the body with its appendages 



