Free-swimming Nauplius of Leptodora hjalina {Lillj.). 211 



Fig. 1 represents the youngest nauplius tow-netted. It should be 

 noticed that Ant. 1 is not a swimming appendage. The posterior end 

 of the body is rounded, as the characteristic caudal forks are not yet 

 developed. The mandible already possesses the rudiment of a biting- 

 blade. The first and second maxilla? are represented by the merest 

 rudiments. Thoracic legs 1-6 are present as conspicuous buds. The 

 lower lip is not yet developed. 



Fig. 1.— -Ventral view of the youngest nauplius. Ant. 2 is relatively much longer 

 than at any other period of life. x 110 diameters. 



On each side of the proctodeum there is a little ectodermal pit 

 secreting a cuticular (V) substance. In an older nauplius, a prominent 

 spine projects out of these sacs, which are then situated at the ends 

 of the caudal forks (fig. 2). These ectodermal pits bear a strong 

 resemblance to the setal sacs of a Chsetopod. 



At this time the mesenteron has an incomplete lumen, but both the 

 stomodseum and proctodeum have reached it. 



Above the gut there is a large collection of yolk-masses surrounded 

 by a membrane of flattened yolk-digesting cells which send processes- 

 inwards between the yolk-masses. There is no yolk-sac duct. 



In an older nauplius the biting blades of the mandibles are more- 



