IO8 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Geologically, the Branchiopoda are as old as the Trilobita, and while they did not have 

 the development in the past that the trilobite had, they were apparently differentiated fully 

 as early. Anostraca, Notostraca and Conchostraca, three of the four orders, are represented 

 in the Cambrian by forms which are, except in their appendages, as highly organized as the 

 existing species. Brief notes on the principal Middle Cambrian Branchiopoda follow : 



Burgessia bella Walcott. 

 Illustrated : Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177, pi. 27, figs. 1-3 ; pi. 30, figs. 3, 4. 



This is the most strikingly like the modern Branchiopoda of any species described by 

 Walcott from the Middle Cambrian, and invites comparison with A pus. The carapace is 

 long, loosely attached to the body, and extends over the greater part of the thorax. The eyes 

 are small, sessile, and close to the anterior margin. 



The appendages of the head consist of two pairs of antennae, and three pairs of slender, 

 jointed legs. Both pairs of antennae are slender and many-jointed, the antennules some- 

 what smaller than the antennae. The exact structure of the limbs about the mouth has not 

 yet been made out, but they are slender, tapering, endopodite-like legs, with at least three or 

 four segments in each, and probably more. 



There are eight pairs of thoracic appendages, each limb having the form of the endopo- 

 dite of a trilobite and consisting of seven segments and a terminal spine. The proximal three 

 segments of each appendage are larger than the outer ones, and have a flattened triangular 

 expansion on the inner side. Walcott also states that "One specimen shows on seven pairs 

 of legs, small, elongate, oval bodies attached near the first joint to the outer side of the leg. 

 These bodies left but slight impression on the rock and are rarely seen. They appear to 

 represent the gills." They are not figured, but taken in connection with the endopodite-like 

 appearance of the segmented limbs, one would expect them to be vestigial exopodites. 



A small hypostoma is present on the ventral side, and several of the specimens show 

 wonderfully well the form of the alimentary canal and the hepatic caeca. The main branches 

 of the latter enter the mesenteron just behind the fifth pair of cephalic appendages. 



Behind the thorax the abdomen is long, limbless, and tapers to a point. It is said to 

 consist of at least thirty segments. 



Compared with Apus, Burgessia appears both more primitive and more specialized. 

 The carapace and limbless abdomen are A pus-like, but there are very few appendagiferous 

 segments, and the appendages are not at all phyllopodan, but directly comparable with those 

 of trilobites, except, of course, for the uniramous character of the cephalic limbs. A closer 

 comparison may be made with Marrella. 



Waptia fieldensis Walcott. 

 Illustrated : Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 181, pi. 27, figs. 4, 5. 



The carapace is short, covering the head and the anterior part of the thorax. The 

 latter consists of eight short segments with appendages, while the six abdominal segments, 

 which are similar to those of the thorax, are without limbs except for the last, which bears 

 a pair of broad swimmerets. The eyes are marginal and pedunculate. The antennules are 

 imperfectly known, but apparently short, while the antennas are long and slender, with rela- 

 tively few, long, segments. The mandibles appear to be like endopodites of trilobites and 

 show at least six segments. As so often happens in these specimens from British Columbia, 



