390 



GEOLOGICAL SUEVEY OF THE TERRITORIES. 



Apus as tlie axis or corm ; and finds that the first and second pairs only 

 are divided iuto joints — the first pair into four joints, and the second 

 into two joints; the remaining pairs not being jointed. The figures in 

 our Plates XVII-XX were drawn, chiefly to exhibit the zoological differ- 

 ences of the appendages in our American Apodidw without reference to 



j the mori)hology of the axis, but j^^ 



! since reading Professor Lankes- 



i ter's su ggestive paper we have re- 



' examined the appendages ; and 



I our observations teach us that, 

 as he states, only the first and 

 second pair of feet show traces 

 of joints, and even these are such T 

 as to be easily overlooked, and 

 should rather be styled pseudo- 

 joints (or pse^ularthra). Plate i \) <^2 '^V /(^■^■■■^■'■■■■■l. 

 XXXI, fig. 4, shows the pseudo- I ]/ \ f [^■■''■^''■■'M 

 segmentation of the axis of the i ' \( W J i WMy^'M 

 first pair of feet in Apus lucasa- 

 mis. 



As our figure indicates, the 

 basal pseudo-joint {ax^) bears the 

 first endite or gnathobase; the 

 second pseudo-joint (ax'^) is in our 



«j-nppip« rpfbif'pd to a, TniTiimnm Fio. 26 —Section of Apns. M. heart ; int, mtesfme; 

 SX>eCie8 leUUCetl to d, lUlumuim, ,j^ ganglion; c. carapace; 1-6, the six exites, 1 being 

 but the second endite rises from t%'suathobase;(7i7Jan<l/;^.tiabellum, representing the 



it; this joint is represented by ®^^*'^^- ^^^^^ lagramatic 

 Lankester as being much larger in the European species. The third 

 joint is tolerably well marked, but its basal limits are not differen- 

 tiated well from the outer part of the first joint. The third endite 

 is thrown off by the third joint {ax^) plainly enough. The fourth joint 

 («iP*) is a definite segment, and from it originate the fourth, fifth, and 

 sixth endites. In A. Incasanus, however, the gill and fiagellum i>]ainly 

 arise from the fourth joint; but according to Lankester's drawing of 

 the same limb in A. cancriformis, these exites arise from the third 

 joint. Taking iuto account, then, the incomplete nature of the two 

 ijasal joints, and the fact that the succeeding pairs of feet are not jointed, 

 we see that they share the nature of the feet in other Phyllopods, and 

 that it is one of the characteristics of the Branchiopods in general, in- 

 cluding the PhyUopoda., not to have truly jointed feet comparable ^vith 

 those of Copepods on the one hand or Malacostracous Crustacea on the 

 other hand. On this account, while it may be safe to regard the basal 

 joint of the anterior foot of Apus as perhaps the homologue of the 

 coxopodite of Decapoda, we should not venture to go farther and homo- 

 logize the succeeding more or less perfected joints with those of the 

 adult Decapodous foot.* 



But the jointed nature of the first foot of Apus and Ltpidurus is valu- 

 able from a morphological point of view, as indicating that the endites 

 are processes from the subjoints, as we may call the imperfectly differ- 

 entiated joints, and do not in any Phyllopod form the joints themselves. 



* Huxley's (Manual of the Anatomy of Invertebrated Animals, 1S77) account of llie 

 nature and homologies of the foot of Lepidurus glacialis is somewhat inaccurate and 

 misleadinj:;. He has torn away the feet represented in his fig. 63 E, F, from the body, 

 leaving the gnathobase attached to the body; and this important and easily recog- 

 nized part is not drawn ; he figures five endites but counts them backwards, beginning 

 with the sixth one. The gnathobase he briefly describes under the name of coxojio- 

 dite. "Each appendage," he says, "consists of three divisions — an endopodite, exo- 



