MOUTH-PARTS OF THE CTPEIS-STAGE OP BALANUS. 273 



traced. Fig. 12 shows the position of the base of the antenna 

 and its socket in the last Nauplius- stage of the Balanus in 

 question. The socket is seen to lie at the side of the labrum, 

 just behind that of the antennule ; the innermost part of the 

 base of the mandible projects as a pivot^ which fits into a slight 

 indentation in the side of the labrum. The origin of the 

 mandibles is well behind the mouth and attachment of the 

 labrum. As the Nauplius prepares for the moult which trans- 

 forms it into the Cypris-stage the body contracts, and many of 

 the structures belonging to this latter stage can be seen beneath 

 the cuticle. If the larva be now carefully dissected out of the 

 cuticle^ or be examined just after the moult, it shows peculiar 

 features in which it approaches the Nauplius-condition, and 

 which are not found in the perfect Cypris-stage. Figs. 9 and 

 10 show the larva in this stage. The edges of the carapace 

 have not yet met in the mid-ventral line, and allow good views 

 of the ventral surface of the animal. The prehensile antennules 

 can be seen at the anterior end of the labrum, which, in most 

 cases, is still recognisable, often as a well-developed structure. 

 The sides of the labrum are indented, as in the Nauplius-stage, 

 by the remnants of the antennae, which are now undergoing 

 histolysis and absorption, but are easily recognisable. Behind 

 these can be seen the mouth-parts of the Cypris-stage already 

 well-formed. These consist of the mandibles with their palps, 

 and the first and second pairs of maxillae. In the more advanced 

 stages the labrum and antennae are small (fig. 11) or have 

 practically disappeared, so that the larva has attained the con- 

 dition (so far as these organs are concerned) seen in the 

 perfect Cypris-stage. A series of stages were obtained showing 

 the gradual reduction of these parts. A highly important 

 feature is that the mandibles, clearly forming with their palps 

 a single appendage, can be detected on the site of the mandible 

 of the Nauplius, behind the antennae and labrum. The labrum 

 shows in some cases more or less clear indications of the 

 median and lateral lobes found in the Nauplius at the distal 

 extremity. It is clear, then, that the palps can neither be 

 regarded as equivalent to the antennae, nor to the lateral lobes 



