126 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



the nostril dorsally. The anterior Up of the slit- 

 Hke mouth of stage 15 moves anteriorly at the 

 midline only, producing a semicircular mouth. 

 Migration of the anterior lip and nostril continues 

 until the mouth lies at the anteroventral extremity 

 and the nostril is at the anterodorsal extremity. 

 In later phases of the stage the mouth enlarges and 

 is hooded by the anterior lip. Oral cirri begin to 

 form posterior to the anterior and posterior lips. 



Movement during this stage is by a rapid undu- 

 lation of the trunk and tail. All ventral flexion 

 of the tail region has disappeared during the trans- 

 ition from stage 15 to 16. 



The anterior boundary of the auricle lies over 

 the center of the ventricle. The flow of blood 

 through the heart and other parts of the prolarva 

 is seen readily. 



In the latter period of this stage the bilateral 

 pigmented retina (eyespot) appears anterior and 

 dorsal to the velum. Melanophores which extend 

 down the length of the neural tube and along the 

 yolk-filled gut begin to migrate ventrally from 

 both levels. Melanophores migrate along the 

 myosepta from the neural-tube level and ventrally 

 from the yolk-somite line. The anterior lip and 

 head region also are well pigmented. 



The appearance of gill clefts marks the initiation 

 of stage 16. They appear and start functioning 

 in order from the first to the seventh. Function- 

 ing of the branchial apparatus can be determined 

 by two means: beating of the velum (between 

 the oral cavity and the first visceral cleft) as it 

 forces water through the pharynx; or, contractions 

 of the gill clefts. The respiratory rate (velum 

 beat) is 120 per minute. 



The end-point of stage 16 is reached when the 

 prolarvae acquire bilateral eyespots and are about 

 to burrow. 



Stage 17: Burrowing (fig. 25) days 17-33 



Size: 7.5 to 9.0 millimeters. 



Gut: Cylindrical and yolk-fUled. Postanal gut 

 absorbed. Esophagus visible on left side. Cloaca 

 transparent. Peristaltic movements in hind gut. 

 Lumen of gut opens at end of stage. 



Mouth: Oval with dorsal and ventral lips. 

 Opens anteriorly. Oral cirri. 



Locomotion: Swimming movements same as in 

 stage 16. Prolarvae burrow. 



Nostril: On dorsal aspect, its ultimate position. 



Transparency: Complete except for yolk-filled 

 gut. 



Figure 25. — Lateral views of stage 17, burrowing, showing the condition of the mouth and the lips. 



