THE CIRRIPEDS. 



409 



The germ cells appear in the undifferentiated blastoderm at a very early 

 period, and later form a conspicuous cluster of cells in the middle of the neural 

 surface of the nauplius, between the separated nerve cords, g.c, in the same posi- 

 tion, therefore, that they have in the polyzoa and pterobranchia. 



In the cirripeds two prominent longitudinal pleural folds are formed that 



Fig. 273. — Limnadia lenticularis. (After NowikofE.) X7 1/2. 



represent the beginning of a two-lobed thoracic shield comparable with that of 

 the ostracodes and other primitive Crustacea, and from which the mantle will 

 develop later. (Fig. 289, mt.) 



In the later stages of the nauplius there is usually developed an enormous 

 labrum (Figs. 7, ro. 289, /.), that overhangs and conceals the mouth, and a promi- 



Fig. 274, — Diagrams of a cirriped larva, illustrating its mode of attachment, revolution, and metamorphosis. 



nent caudal lobe, a.l., with the anus on its hsemal side. An adhesive disc forms 

 near the apex, or the haemal surface of the head, by which the larva, for a longer 

 or shorter period, is attached to foreign objects. 



A nauplius-like larva, with most of the characters indicated above, occurs 

 under various disguises and modifications in all the subphyla of the acraniates. 

 In its modified form we shall refer to it as the naupula. 



