THORACICA. 213 



DOHRN (No. 42) and WILLEMOES-SUHM (No. 62), that these pairs 

 of spines correspond to the rudiments of the six pairs of thoracic 

 limbs in the Cypris-like larva appears to us somewhat plausible. 

 This last stage would then have to be assumed to be the Meta- 

 nauplias stage. 



For more recent observations on the larvae of the Lepadidae we are above 

 all indebted to WILLEMOES-SUHM (No. 62). DOHRN (No. 42) has minutely 

 described a large Nauplius larva probably to be referred to Lepas australis 

 under the name of Archizoaea gigas. These stages have, however, absolutely 

 nothing to do with the Zoaea larva of the Malacostraca. The development of 

 long spine-like processes is a protective adaptation often occurring in pelagic 

 animals (cf. the skeletons of the Radiolaria and the spines of the Pluteus larvae). 



In the whole series of Nauplius stages, except the last, there is no 

 essential alteration apart from the increase in size of the body and 

 the above-mentioned development of thorn- and spine-like processes. 

 In the last stage which immediately precedes the Cypris stage 

 (Metanauplius, Fig. 102 B\ important new rudiments of organs 

 appear. With respect to the general form of the body, it should 

 be emphasised that the lateral parts of the dorsal shield now already 

 begin to bend downwards, and, covering the body laterally, fore- 

 shadow in position the valves of the shell in the Cypris stage. 

 The three anterior pairs of limbs still bear a general resemblance to 

 the typical Nauplius limbs, but already, within the first pair (first 

 antennae), the rudiment of the sucking disc so important in the 

 Cypris stage can be made out (KROHN, No. 50; WILLEMOES-SUHM, 

 No. 62 ; GLAUS, No. 8). This fact affords a direct proof that the 

 adhesive antennae of the later stages are actually derived from 

 the most anterior pair of Nauplius limbs (KROHN, No. 50 ; FR. 

 MULLER). Behind the mandible is to be seen the slightly developed 

 rudiment of the fourth pair of limbs ; it is, however, probable, as we 

 shall see further later, that we have here the rudiments of two con- 

 secutive pairs of limbs (mx) (METSCHNIKOFF, No. 53 ; GLAUS, No. 8). 

 The thoraco-abdominal region of the larva has increased considerably 

 in size; in it, beneath the Nauplius cuticle, can be recognised the 

 rudiments of the six pairs of swimming limbs (/ r -/ r/ ) of the larva 

 (thoracic limbs), as well as the caudal processes of the short abdomen 

 (furcal processes) (KROHN, No. 50 ; GLAUS, No. 8). Another im- 

 portant point is that in this stage, near the Nauplius eye, the paired, 

 movable compound eyes (Fig. 102 B) already appear as rudiments. 

 In this Metanauplius stage, therefore, all the more important organs 

 and limbs of the Cypris-like larva are formed, and the larva itself, 



