4-08 GEOLOGICAL SUEVEY OF THE TEKRITOEIES. 



and an outer or respiratory portion, as in JSfehalia and Decapoda. The 

 endopodite of Liinulus [en) is axially jointed, tliere being three well- 

 marlied joints to this part of the limb. The branchiate portion of the 

 limb (ex) is homologous with that of Nebalia, and the epipodital or 

 branchiate portion of the Decapod thoracic limb. At the same time 

 that of Limulus i:>resents some remarkable peculiarities, *. e., the exo- 

 podal (or epipodital) portion is jointed ; and the gill, instead of being a 

 simple tan-like extension, as in the Phyllopoda and Phyllocarida, is re- 

 placed by a number of flat, thm gill-plates, arranged i)arailel to each 

 other, in an antero-posterior sense. When, however, we compare the 

 gill, or rather the epipodital portion of the leg of Limulus, with that of 

 the lobster we have the various fundamental elements, *. e., an artery 

 and a vein passing into the foot and in connection with a number of 

 gill-13lates. In the lobster we have along the base of the gill (fig. 33) 

 collective veins and an artery into which the blood passes after being 

 aerated in a large number of cylindrical gill-filaments. Morpholog- 

 ically there is a fundamental resemblance between the two types of 

 branchiae; in Limulus there are gill-plates, in Decapods gill-filaments, 

 each presenting in the aggregate a large respiratory surface. The gills 

 of the Isopoda are in some degree intermediate between the Decapods 

 and the Merostomata. 



When we compare the anterior or cephalic appendages with the 

 thoracic appendages of the lobster, there is a close resemblance in the 

 axially -jointed endopodite (fig. 38, end) of Limulus with its large terminal 

 claw to the foot of tbe Decapod. The absence of the gill or branchiate 

 (epipodital) portion in Limulus is correlated with the ambulatory nat- 

 ure of its anterior or cephalic appendages. 



In the trilobites, however, as may be seen by Mr. Walcott^s able res- 

 toration (fig. 40), we have attached to the thoracic ambulatory feet 

 a respiratory epipodital portion. In some respects, then, in the trilo- 

 bites we have a style of structure intermediate between the Merosto- 

 mata and the Decapoda. 



In the trilobite we ai)parently have, besides a true-jointed locomotive 

 endopodite (fig. 40, en)^ an inner jointed appendage (ew^), which may 

 be homologized with the exopodite of the Decapod maxillipede (fig. 

 33). From near its base arises the two singular spiral gills, which are 

 unique. It is to be observed that the two jointed appendages and the 

 stem of the gills arise from what appears to be a true coxopodite, and 

 that this coxopodite is ai)parently homologous with that of Limulus 

 (fig. 38). It thus appears that a study of the general internal anat- 

 omy and of the appendages of the normal, recent Crustacea (Neocarida) 

 throws light upon the structure of the archaic Crustacea {Palwocarides)y 

 and that the most archaic Keocarida, the Phyllocarida (Nebalia), as re- 

 gards their thoracic limbs, do not remotely resemble the abdominal 

 limbs of Limulus. In this connection we would draw attention to fig. 

 39, which is designed to show the possible relations between Limulus 

 and Calyraene or the Merostomata and the Trilobita. The essential 

 difference is in the nature of the limbs; the thoracic limbs of the trilo- 

 bite, while having a jointed endopodite as in Limulus, also having an 

 exopodite and a forked spiral gill. Now, if we append to the coxopo- 

 dite of Limulus an exopodite, and instead of having the gills arranged 

 anteroposteriorly, like the leaves of a book, have them arranged on one 

 side (the outer) of a more or less cylindrical epipodite, as we have 

 drawn them in fig. 39, we shall hardly be tloing greater violence to 

 nature than we see to occur in any Decapod, where, as may be seen in 

 fig. 3a of the lobster, the maxillae have no sijecialized exopodite, such 



