48 SCIENCE PROGRESS. 



and persisted for a time in far greater number than in the 

 Trilobites. The limbs further remained phyllopodan as 

 swimming plates. While, however, thus retaining primitive 

 conditions in the trunk, the limbs of the anterior or head 

 segments underwent a very marked specialisation. The 

 first pair remained as simple antennae, the second pair de- 

 generated and lost their jaw plates, but ultimately developed 

 their sensory functions to become the second antennae of 

 the typical Crustacean. The third pair lost their locomo- 

 tory portions and became the powerful mandibles working 

 between the labrum and labium (or labial lobes). The two 

 following pairs also more or less completely lost their 

 locomotory functions, the basal portions persisting as ac- 

 cessory jaws, the maxillae. This formula for the head limbs, 

 seen in its most primitive condition in Apus, became from 

 henceforth the typical mouth formula of the Crustacea. 



From such a form as this, all the modern Crustacea 

 can be deduced. 1 The segmentation became more 

 specialised and consequently less rich, the limbs lost their 

 primitive phyllopodan characters, the two pairs of antennae 

 travelled forwards to the anterior edge of the head ; the 

 mouth and mouth-parts also moved forward, causing the 

 almost complete obliteration of the great labrum. The 

 metastomial lobes persist as the " paragnathes," while the 

 great fleshy mandibles of Apus became hard chitinous 

 plates. The degenerate locomotory portions of the maxillae 

 became the palps, while, in some forms, a certain number 

 of trunk limbs, the ventral branches of which, in the primi- 

 tive form, served to push food forward towards the mouth, 

 move forward and join the maxillae, as accessory jaws, the 

 maxillipedes. The great carapace has undergone many 

 variations ; in some cases it has degenerated, in others it 

 has given rise once more to a system of pleurae along the 

 trunk by segmental repetition, and forms such as the 

 Isopods have arisen, closely resembling the Trilobites, and 

 perhaps to some extent taking their place in the modern 

 seas. 



1 Cf. on this Prof. K. Grobben, Sitzungsber. d. K. Akad. Wien, vol. 

 ci., 1892, pt. i. 



