354 ME. H. M. BEKNAKD ON THE SYSTEMATIC [Aug. 1 895, 



This is no longer the case in Apus. But, as I have endeavoured 

 to show, the conditions there found can be traced in detail from 

 such a hypothetical primitive condition. The head-limbs, in de- 

 veloping their jaw-pieces, did not require the dorsal portions of 

 the parapodia ; these, however, in some cases persist as vestiges. 

 On the trunk, on the other hand, the dorsal portions of the parapodia 

 were most needed and developed as locomotory appendages. 



Here again, then, in this primitive homogeneity of the head- and 

 trunk- appendages, Triarihrus ranks far below Apus, in which certain 

 of the appendages near the mouth are already structurally modified 

 for co-ordinated action as jaws and maxillae, and accordingly form a 

 group apart from the thoracic limbs. 



Dr. Beecher's figures seem to show a tendency in the jaw-pieces- 

 of the head-limbs of Ti'iarthrus to become more and more powerful 

 as we recede from the mouth ; those of the fifth pair appear as the 

 largest {A, p. 353). This cannot be considered as a mere accident,, 

 inasmuch as we find the same gradual increase in the size of the jaws 

 distinctly marked in Limulus (C, p. 353), and enormously pronounced 

 in Eurypterus. In these two animals the maximum of size is 

 reached in the sixth pair of appendages. According to the browsing 

 annelid theory, the pair of jaw-pieces nearest the mouth (that is, of 

 the second appendages) would, one would think, develop to be the 

 largest. Yet in no case does this happen. It is, however, typical 

 of many chaetopods that the first pair of appendages is reduced to 

 little more than the cirrus of the notopodium ; the second pair, com- 

 plete as far as form goes, is often small, while the third and follow- 

 ing pairs increase in size until a maximum is reached. It is 

 obvious, then, that if such a chsetopod took to browsing in the 

 manner described, the limbs which were first ranged on each side 

 of the mouth would necessarily show this gradual increase in 

 size. 



On this primitive condition the arrangement of the jaws in Apus 

 (B, p. 353) is a striking advance in specialization. We there find 

 that the smaller jaw-pieces of the second pair of appendages were 

 allowed to atrophy in favour of the larger and more powerful jaw- 

 pieces of the third appendages, which became true biting-jaws 

 working within the mouth-aperture — that is, between the labrum 

 and the labial lobes. This arrangement amounted to a practical 

 monopoly of the mouth-aperture by a single pair of mandibles ; 

 while the following pairs, divorced from any close connexion with 

 the mouth, and being, compared with the mandibles, of secondary 

 importance, show either no great specialization or even degenera- 

 tion (B, 5, p. 353). 



This striking specialization of the mouth-parts found in Apus 

 must have had some advantages over the simpler and more primi- 

 tive (annelidan) condition found in Triarthrus and Limulus. It is 

 not improbable that the single pair of highly specialized jaws, their 

 cutting edges working between the labrum and the labium — that is, 

 well within the mouth-aperture — are in many respects far more 

 efficient instruments for alimentation on which growth and multi- 



