702 



ARTHROPODA 



PHYLUM VII 



podites are jointed, crawling legs ; posteriorly, especially on the pygidium, 

 the joiiits become flattened and leaf-like, carrying tufts of setae, and being 

 adapted for swimming. . 



The exopodites are fringed along their posterior edges with narrow, 

 oblique lamellar elements becoming filiform at the ends, thus Converting the 

 limb into a swimming organ, and probably also serving respiratory functions 



(Figs. 1343, 1346). ^ . . i.^ -,. . 



Hahits.—\n the absence of any closely allied recent forms, it is difficult to 

 reach definite conclusions respecting the manner of life of Trilobites, except 



Fig. 1346. 



Cryptolithus tessellatus Green. Utica Shale (Ordovician) ; Rome, New York. A, Left half of pygidium and 

 three thoracic segments, with test removed, and showing fringes of the exopodites. B, Ventral aspect of same. 

 tt, Endopodite ; b, Exopodite. lo/i (after Beecher). 



such as are based upon their Organisation and mode of occurrence. They were 

 undoubted marine animals, since their remains are found only in salt-water 

 deposits, associated with brachiopods, cephalopods, crinoids, and other typical 

 oceanic forms. Some species are plentiful in calcareous or argillo-calcareous 

 deposits, with thick-shelled brachiopods, gastropods and reef-building corals, 

 which evidently did not live at any considerable depth. Other forms appear 

 to have been bottom crawlers, frequenting either muddy or sandy bottoms ; 

 and again, others like Cryptolithus, lived partly buried in the soft mud. On 

 the other band, many species indicate, from the absence of visual organs, a 

 comparatively deep-water habitat. The structure of the appendages of many 

 was probably such as to permit of both swimming and 

 crawling, as in a number of families of modern Crustacea, 

 and they were therefore restricted neither to the shore 

 nor to the bottom. This doubtless explains the occur- 

 rence of the same species in very different Sediments. 



Power of Enrollment.— The bodies of most Trilobites 



were capable of being roUed up completely like many 



Fia 1.347. ^^ ^he Isopods (Fig. 1347). In the enrolled condition 



caiyimne mseH Foerste. ^^^ m^rgin of the pygidium is closclv applied to the 



SSXlSr"'^'''^''''^- doublure of the cephalon, thus entirely concealing the 



ventral side of the body. The thoracic segments over- 



lap, and admit of more or less motion upon one another. The pleura also 



