DEVELOPMENT OF EUPHAUSIA SUPERB A 83 



segments ; it also shows the natural flexion of the limb which is typical of thoracic ap- 

 pendages from this stage onward to the adult. Between Th. VI and the first abdominal 

 segment, in the larva having five pairs of setose pleopods, there is a small process on 

 each side, which is flattened antero-posteriorly and is the vestige of Th. VII (Fig. 19 a). 

 In this aspect it appears roughly semicircular in outline with a small emargination near 

 the external border. The ill-defined outer lobe is the developing gill, while in the inner 

 part is the luminous organ belonging to this appendage which will develop in the 

 next stage. 



LN \ s 



'^ 



VI 



" I II III IV V 



Fig. 19. Second Furcilia (continued). 



a, lateral view of bases of thoracic limb VI and pleopod I with, between them, thoracic limb VII, a Th. 



VII seen from behind ( x 83); 



b, gills on thoracic limbs II-VI in larva with 4 setose, i non-setose pleopods ( x 83); 



c, gills on thoracic limbs II-VI in larva with 5 setose pleopods ( x 83); 



d, pleopods I-V in larva with 3 setose, 2 non-setose pleopods ( x 35); 



e, pleopods I-V in larva with 4 setose, i non-setose pleopods ( x 35); 

 /, pleopods I-V in larva with 5 setose pleopods (x 35); 



g, appendix interna ( x 360); h, setae within the integument of a non-setose pleopod ( x 165). 



Gills. In the smaller larvae included in this stage the gills are at much the same stage 

 of development as in the larva having five pairs of non-setose pleopods, the only ad- 

 vance noted in the larva having three pairs of setose pleopods is that the gill of Th. V is 

 now bilobed. In the larva with four pairs of setose pleopods the branchiae of Th. II-VI 

 are all distinctly bilobed (Fig. 19 b). The gills of Th. II-VI in the larva with all pleopods 



