MOUTH-PAUTS OF THE 



PAL EMONID 



PRAWNS. 



49 



7. The relation of the parts of the biramous limb to those of 

 its leaf-like forerunner may be elucidated by a study of actual 

 phyllopod appendages. It would seem that the nabellum is 

 represented by the exopodite, and the main axis, ending in the 

 apical lobe, by the protopodite and endopodite. In regard to the 

 exopodite, the evidence of the Leptostraca, in which it is flattened 

 and bears just the same relation to the other parts of the limb as 

 the nabellum, is too strong to be set aside without more con- 

 vincing reasons than have been adduced. Theories which 

 disregard this consideration depend upon the precarious support 

 afforded by a comparison of the arrangement in various cases of 

 the endites. These structures, however, are very variable and 

 difficult to homologize, and in particular those of the maxilla, 

 which in adult reptant Decapoda appear to suggest that the fifth 

 and sixth endites have become endopodite and exopodite, have in 

 more primitive members of the same group a quite different 

 aspect, which supports strongly the theory stated above. The 

 larvae of. various Carides, as, for instance, that of Palcemonetes 

 (text-fig. 18), show at the base of the endopodite of the maxilla, 

 in the region of the iscTiipodite, a distinct fifth endite. In other- 

 cases, as in Anisocaris (text-fig. 17) and Penceits (text-fig. 16), 



axis of the phyllopod limb, and in shape move nearly resembles the larval maxilla 

 of a Decapod than the trunk-limb of a Branchiopod. It is remarkable only for the 

 comparative^ unimportant facts that the region of the third and fourth endites, 

 instead of forming a single joint (the basipodite), is divided by an articulation infj 

 two, each of which bears one endite, and that beyond the fourth endite the limb ha3 

 but four segments, instead of the five which the maxilla of Gerataspis (text-fig. 15) 

 indicates as the full complement of the corresponding region of the decapod limb. 

 Jt does not seem impossible that an additional segment ma} 7 - eventually be 

 discovered at the end of this appendage. The conditions are none too clear in 

 Beecher's model. The " exopodite," however, stands upon the segment of the 

 second endite, not upon that of the third and fourth. Probably this indicates that 

 the structure here called the exopodite is not homologous with that to which the 

 same name is applied in recent Crustacea, but is a modified epipodite. In that 

 connection it is interesting to note that Beecher attributes to it a respiratory 

 function. If the foregoing theory be correct, the thoracic limbs of Trilobita present 

 a very remarkable analogy to those of Decapoda, consisting as thej r do of a sub- 

 cylindrical, seven-jointed axis with a complex respiratory structure borne on the 

 outside near the base. Unlike the podobranchs of the Decapoda, however, the 

 epipodite of the Trilobita " contrives a double debt to pay" as gill and "exopodite.' 



The foregoing interpretation of the limb of Triarthrus receives ver}' interesting 

 support from the arrangement of the parts of the mesosomatic appendages of 

 Limtdus. Here the axis bears on its outer side three broad exites, separated from 

 it and from one another by sutures to which correspond notches on the edge ot the 

 limb. (The first exite of the gill-bearing limbs is, upon its anterior face, divided 

 into about a dozen strips- by less-marked sutures, of which only two remain in the 

 genital operculum.) That part of the axis which bears the first and second exites is 

 inijointed, but the third is attached to a well-marked segment, and beyond this two 

 free joints form the apex of the lirnb. The first and second exites are crossed 

 obi quely upon their hinder face by the gill-lamellae. It is pretty clear that they 

 are epipodites, and a comparison at once suggests itself between them and the very 

 differently shaped "exopodite" which bears gill-lamelhe in Triarthrus. The third 

 exite in Limulus, sharply distinct from the second, is no doubt the exopodite. 



The supposed biramous mouth-parts of Insecta are not so in reality, and I take 

 this opportunit) r of retracting the view I haye expressed on that point in my 

 'Manual of Zoology' (London, 1912). The lacinia and galea are, I now believe, 

 the third and fourth endites, and the palp is the endopodite. 



Proc. Zool, Soc, — 1917, No. IY. I 



