48 



MR. L. A. BORRADAILE ON THE 



mastication developed the gnathobase with or without some of the 

 other endites, those for respiration the epipodites, and those for 

 contact with large objects the main axis. Generally speaking - , the 

 limbs which in the adult are adapted to other functions than 

 swimming are in the larva (and were perhaps at one time through- 

 out life) natatory, and retain in their later condition traces of the 

 biramous plan, so that they may be regarded as belonging to the 

 biramous type. In becoming biramous, the limb has lost its leaf- 

 like character. This has in all cases befallen the antennae and 

 mandibles, which, precociously adapted in the Nauplius to swim- 

 ming, never exhibit the primitive configuration. On the other 

 hand, the maxilhe almost invariably retain a good deal of resem- 

 blance to the phyllopod prototype, probably because their position 

 makes it impossible for them to be of much use either as jaws or 

 for swimming. The trunk-limbs of Branchiopoda are phyllopod, 

 and show in some detail the features which it is necessary to 

 attribute to those of the ancestral crustacean * ; those of the 

 other groups are biramous f , with the exception of the thoracic 

 appendages of the Leptostraca, which exhibit various degrees of 

 transition from the phyllopod to the biramous condition. 



* The lobes of the trunk-limbs of Branchiopoda (text-figs. 3-5) are exceedingly 

 difficult to homologize. It seems, for instance, quite possible that the Anostraca 

 have no flabellum, and that the structure known by that name which is jointed to 

 the end of their limbs corresponds to the so-called last. (sixth) "endite" of Apus and 

 Limnadia. If so, however, an additional endite must be intercalated into the series 

 in Anostraca or excalated from it in Notostraca, for in the former group there are 

 six of these lobes before the terminal structure in question, but in the latter only 

 five. The facts of meristic variation would explain this discrepancy without 

 difficulty, but the actual solution is probably indicated by the presence in Apus, 

 though barely in Lepidurus, of what looks like a vestigial endite, standing in the 

 gap between the first and that which has been regarded as the second of the series. 

 This little lobe bears no bristles, but the section of the axis opposite it is defined by 

 lines of soft cuticle, such as those which mark out the segments of the other endites. 

 If there be here a true endite, the series in Apus corresponds with that of Anostraca. 

 Lepidurus must then be considered to have lost the second endite (though not its 

 segment), and in Limnadia it has perhaps fused with the third. It is true that 

 this hypothesis is open to the objection that it brings the epipodite opposite the 

 third endite, but such a displacement presents no great difficulty in view of the 

 vestigial nature of the supposed second segment, which does not reach the outer 

 edge "of the limb, and of the fact that the epipodite is attached not wholly to the 

 third segment, but astride of the articulation between it and the portion of the limb 

 proximal to it. Again, the little process known as the "subapical lobe," present in 

 Apus and Branchipus, but barely or not at all represented in some other genera, as 

 Chiroceplialus and Limnadia, may or may not represent the true end of the axis 

 of the limb. If it does not, then the apex must be the so-called last endite. This 

 (if the above conjecture with regard to the fhibellum of Anostraca be true) is always 

 an organ sui generis, separated by a more or less distinct joint from the re>t of the 

 limb. In either case, the condition of the apical lobe in the Branch ipoda is 

 abnormal as compared with that which it shows in the larval maxilla of the 

 Decapoda. and in Triarthrus, which in this respect prohably more nearly represent 

 the ancestral Crustacea. To judge from the thoracic limbs of Nebalia (text-fig. 9), 

 whose end-joint differs strongly from the rest and is much more sharply articulated, 

 the appendages upon the trunk of the phyllopod ancestor of the Malacostraca must 

 have resembled those of the Branchiopoda in respect to the apical lobe. 



f The limbs of the Trilobita (text-figs. 6, 7) present an interesting problem in 

 that, while they are undoubtedly biramous in the sense of consisting of two suhequal 

 rami upon a common stem, their " exopodites " are inserted in a different position 

 from those of recent Crustacea or from the flabella of the Branchiopoda. The 

 "endopodite" of the pygidial limb (in Triarthrus) is clearly comparable with the 



