142 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



from the Hypoparia, continuing in the pelagic habitat. At first, most of the trilobites seem 

 to have led a crawling existence, but about Middle Cambrian time they began to go back par- 

 tially to the ancestral swimming habits, and retained some of the trunk segments to form 

 a larger pygidium. The functional importance of the pygidium explains why it can not be 

 used successfully in making major divisions in classification. Nearly related trilobites may 

 be adapted to diverse methods of life. 



EVOLUTION WITHIN THE CRUSTACEA. 



The question naturally arises as to whether the higher Crustacea were derived from 

 some one trilobite, or whether the different groups have been developed independently from 

 different stocks. The opinion that all other crustaceans could have been derived from an 

 Apus-\ike form has been rather generally held in recent years, but Carpenter (1903, p. 334) 

 has shown that the leptostracan, Nebalia, is really a more primitive animal than Apus. He 

 has pointed out that in Leptostraca the thorax bears eight pairs of simple limbs with 

 lamelliform exopodites and segmented endopodites, while the abdomen of eight segments has 

 six pairs of pleopods and a pair of furcal processes, so that only one segment is limbless. 

 Contrasted with this are the crowded and complicated limbs of the anterior part of the 

 trunk of Apus, and the appendage-less condition of the hinder portion. Further, a compari- 

 son between the appendages of the head of Nebalia and those of Apus shows that the former 

 are the more primitive. The antennules of Nebalia are elongate, those of Apus greatly re- 

 duced ; the mandible of Nebalia has a long endopodite, and Carpenter points out that from it 

 either the malacostracan mandible with a reduced endopodite or the branchiopodan mandible 

 with none could be derived, but that the former could not have arisen from the latter. 

 The maxillae of Apus are also much the more specialized and reduced. 



Nebalia being in all else more primitive than Apus, it follows that the numerous ab- 

 dominal segments of the latter may well have arisen by the multiplication of an originally 

 moderate number, and the last trace of primitiveness disappears. 



It is now possible to add to the results obtained from comparative morphology the testi- 

 mony of palfeontology, already outlined above, and since the two are in agreement, it must 

 be admitted that the modern Branchiopoda are really highly specialized. 



As has already been pointed out, Hymcnocans, the leptostracan of the Middle Cambrian, 

 has very much the same sort of appendages as the Branchiopoda of the same age, both 

 being of the trilobite type. Which is the more primitive, and was one derived from the 

 other? 



The Branchiopoda were much more abundant and much more highly diversified in 

 Cambrian times than were the Leptostraca, and, therefore, are probably older. Some of the 

 Cambrian branchiopods were without a carapace, and some were sessile-eyed. These were 

 more trilobite-like than Hymcnocans. Many of the Cambrian branchiopods had developed 

 a bivalved carapace, though not so large a one as that of the primitive Leptostraca. The 

 present indications are, therefore, that the Branchiopoda are really older than the Leptos- 

 traca, and also that the latter were derived from them. It seems very generally agreed that 

 the Malacostraca are descended from the Leptostraca, and the fossils of the Pennsylvanian 

 supply a number of links in the chain of descent. Thus, Pygoccphalus cooperi, with its 

 brood pouches, is believed by Caiman (1909, p. 181) to stand at the base of the Peracaridan 

 series of orders, and Uronectes, Palccocaris, and the like are Paheozoic representatives of the 

 Syncarida. Others of the Pennsylvanian species appear to tend in the direction of the Sto- 



