42 



THE APODID^ PARTI 



pairs of limbs of the Crustacean head, so marked off 

 for all time by the bending round of the five anterior 

 segments of the original Annelid, and the growth of 

 the shield. The three posterior limbs develop the 

 ventral branches of the original Annclidan parapodia 

 as masticatory apparatus round the mouth ; the dorsal 

 branches are more or less completely degenerated, 

 reappearing, when the antennae adopt a frontal 

 position, as palps, or palp-carriers. This arrange- 

 ment of masticatory ridges may have had some- 

 thing to do with the preservation of the modern 

 Crustacea, while the older groups with other, and 

 probably less advantageous, combinations, such as 

 those developed by the Trilobites, have died out. 



We find here also the origin of the rule that, among 

 the Crustacea, the dorsal parapodia are less developed 

 on the mandibles than on the first maxilLne, and less 

 on the first than on the second maxilLne. The typical 

 Crustacean mandibular palp consists of the dorsal 

 parapodium carrying its sensory cirrus, both apparently 

 being sensory organs. 



The limbs liable to most modification are, naturally, 

 the first two, i.e. the two pairs of antennae. In Apus 

 we find the first pair retaining perhaps the original 

 size, the second pair, however, degenerating. In 

 Limulus both pairs are. well developed as chelate 

 seizing limbs, the second even redeveloping its ventral 

 parapodium as a masticatory ridge. In the Euryp- 

 teridae, other characteristic changes will be noticed, 

 and where possible will be traced to changes in their 

 manner of life. We may here perhaps suggest the 



