1 62 THE APODID^E PARTI 



The development of the shield is well shown in 

 Figs. 35 and 36 of the Nauplius of L. productus, which 

 should be compared with the commencement of the 

 development of such a shell in the Trilobite Acid- 

 aspis, Fig. 48, p. 215, the neck lobe of which, developed 

 as a thorn-carrier, suggests a very probable origin of 

 the dorsal shield. We see it again in the Nauplius of 

 A. cancriformis, Figs. 37 and 38, developing as a 

 fold. 



No great difficulty need be found in the fact that 

 the fold of the fifth segment should appear in the 

 Nauplius before any trace of the limbs of the fourth 

 and fifth segments, i.e., of the two pairs of maxillae. It 

 is doubtless of considerable advantage to the larva to 

 develop the shield as early as possible as a protective 



covering. 



4. The Upper Z,//.--The labrum is another very 

 characteristic feature of most Nauplii. In some, 

 indeed, it reaches an enormous size (see Fig. 37). 

 The homologies of this organ have been a great 

 puzzle to zoologists. Packard suggests the median 

 frontal tentacle of certain larval Annelids. Its 

 presence in the Nauplius seems certainly to suggest 

 that it was a prominent organ in the racial form of 

 the Crustacea, especially as it is difficult to see what 

 special advantage it can offer to the larva as such. 

 Our derivation of Apus and of the whole class of 

 Crustacea from a bent Annelid, homologises it, as 

 already described, with the Annclidan prostomium, 

 which is probably the most important of all the 

 external organs of the Annelidan body. Its general 



