Dr. W. T. Caiman — The Appendages of Trilobites. 363 



all the main, divisions of the Crustacean stock, and the rami are 

 filiform and multiarticulate not only in the Notostraca (Aptis, etc.) 

 but also in certain Cirripedes. Nothing of the kind is found in any 

 of the Arachnidan groups. 



While giving due weight to these and other evidences of 

 Crustacean affinity, it is important not to lose sight of the characters, 

 some of them of considerable weight, in which the Triiobita differ 

 from what we must suppose the primitive Crustacean to have been 

 like. One of the most important is the absence of a carapace. In 

 the Crustacea the carapace is formed by a fold of the dorsal integu- 

 ment arising from the hind margin of the head-region, enveloping 

 and sometimes coalescing with more or fewer of the body-somites. 

 Only in the Anostracous Branchiopoda, in some Syncarida 

 (Bathynella), and possibly in the Copepoda, is this shell-fold entirely 

 absent, and it is a reasonable conclusion that it must have been 

 present in the ancestral stock of the Crustacea. No Trilobite 

 shows any trace of such a fold. 



The sessile eyes of the Trilobites may perhaps be reckoned as 

 another non-Crustacean character. Sessile eyes are indeed common 

 enough among recent Crustacea, but there are good reasons for 

 thinking that the condition is in all cases a secondary specialization 

 and that the eyes were primitively pedunculate and movable. Even 

 the sessile eyes of the Notostraca and Conchostraca, which are 

 movable and covered by an invagination of the integument, are 

 regarded, with' considerable probability, as derived from stalked 

 eyes. In the Triiobita we have no evidence that the eyes were ever 

 movably pedunculate, although the suggestion has been made that 

 the eye-bearing " free cheeks " may have been formed by the 

 expansion of ocular peduncles. 



Finally, it is to be noted that the exopodites of Triarthriis with 

 their fringe of lamellar elements, possibly branchial in function, are 

 something very different from the " setiferous exopodites " of many 

 Crustacea with which they have been compared. It is possible that 

 we have here a hint as to the origin of the " gill-books " of Limulus. 



No Trilobite yet discovered is so generalized that it could be 

 regarded as standing in the direct line of descent of either Crustacea 

 or Arachnida, but the group as a whole seems to point the way 

 towards some form that may have been the common ancestor of both. 

 Dr. Walcott's wonderful discoveries in the Burgess shale give hope 

 of further progress in tracing the phylogeny of the primitive 

 Arthropods, and the fuller investigation which he promises of the 

 remarkable llarrella will be awaited with much interest. 



EXPLANATION OF PLATE VIII. 



FlG. 1. — Neolenus serratus (Rominger). Burgess Shale (Middle Cambrian), 

 British Columbia. (From Walcott.) Specimen displaying the ventral 

 surface with the appendages in situ. At the posterior end of the 

 body a portion of one of the caudal rami is visible. (From 

 a retouched photograph, xl|.) 

 ,, 2. — Diagram of a transverse section of tbe body. d.s. dorsal shield ; 

 ep. epipodite ; ex. exopodite ; exi. exite ; int. intestinal canal ; 

 pr. gnathobasic process of protopodite ; v.i. ventral integument. 



