TBILOBITA <:i7 



the joints become flattened HIM! leaf like, carrviim tufts of setae, and b 

 adapted for swimming. 



The exopodite* arc fringed alon.u' their pn>i-rinr rd.urs with narr.\\. 

 oblique lamellar elements becoming filiform at the end.-. thu> cmm-i t in:: Jin- 

 limb into a swimming organ, and also serving respiratorv functions (Figs. 

 1267, 1270). 



Habits. In the absence of any closely allied recent forms, it is difficult to 

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

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

 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 fnu> appear 

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

 and again, others like Triiim-lma, lived partly buried in the soft mud. On the 

 other hand, many species indicate, from the absence of visual organs, a com- 

 paratively 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 occurrence of the same 

 species in very different sediments. 



Power of Enrolment. The bodies of most Trilobites were capable of being 

 rolled up completely like many of the Isopods (Fig. 1271). In the enrolled 

 condition the margin of the pygidium is closely applied 

 to the doublure of the cephalon, thus entirely concealing 

 the ventral side of the body. The thoracic segments 

 overlap, and admit of more or less motion upon one 

 another. The pleura also imbricate, and their fulcra 

 are provided with facets upon which the fulcra of 

 adjacent segments impinge. The ends of the pleura 

 thus protect the ventral surface along the sides, when 

 the animal is enrolled. Some forms appear to have 

 possessed the power only to a limited degree. In 

 these, the creature is usually found extended, and the Enroiic.i 

 facets on the fulcra are either rudimentary or absent. 



Ontogeny. Minute spherical or ovoid fossils associated with Trilobites 

 have been described as possible Trilobite eggs, but nothing is known, of 

 course, of the embryonic stages of the animals themselves. The smallest and 

 most primitive organisms which have been detected, and traced by means of a 

 series of specimens through successive changes into adult Trilobites, are little 

 discoid or ovate bodies not more than 1 mm. in length. This first larval form 

 has been named the protaspis (Beecher), arid has been found to be the normal 

 larval type characteristic of all Trilobites. It is believed to approximate the 

 protonauplius form, or the theoretical, primitive, ancestral, larval form of the 

 Crustacea. 



The simple characters possessed by the protaspis are the following, as 

 drawn from the study of this stage in all the principal groups of Trilobites : 

 Dorsal shield minute, m>f ///<>r<' /////// 0'4 f<> 1 nun. in ////// ; ciri-nlnr "/ "'"/' i,i 

 form ; axis distinct, more or less sti-mi'ili/ rf///////r/W. I hit if'.-'/ Inj Imitfifm/imd g\ 



