FEEDING. 



103 



chiefly in very fine-grained shales, in company with graptolites. These latter are distributed 

 by currents over great distances within short periods. It is somewhat curious that the nearly 

 blind burrowing Trinucleidae, the dysphotic, large-eyed Remopleuridse and Telephus, the blind 

 nektonic Agnostidse and Dionide, and the planktonic graptolites should go together and make 

 up almost the entire fauna of certain formations. Yet, when the life history of each type 

 is studied, a logical explanation is readily at hand, for all have free-swimming larvae. 



A list of the methods of life noted above is given by way of summary, with examples. 



Pelagic 



Benthonic < 



Planktonic 



Nektonic 



Crawlers and 



slow swimmers 

 Crawlers and 



active swimmers 

 Crawlers, slow 



swimmers, and 



burrowers 



( Primarily 

 (I Secondarily 

 "Primarily 



Secondarily 



Earliest protaspis of all trildbites 



Deiphon, Odontopleura, etc. 



Later protaspis of all trilobites. Naraoia 



Probably many thin-shelled trilobites. with large pygidia 



(only partially nektonic) 

 Cyclopygidae ) . 

 .RemoplLida ( ("ekton.c dysphot.c) 



Most trilobites with small pygidia. Triarthrus, Para- 



doxides, etc. 

 Most trilobites with large pygidia. Isotelus, Dal- 



manites, etc. 



Trinucleidse, Harpedidae, some Mesonacidae, etc. 



FOOD AND FEEDING METHODS. 



This subject has been less discussed than the methods of locomotion. The study of 

 the appendages has shown that while the mouth parts were not especially powerful, they were 

 at least numerous, and sufficiently armed with spines to shred up such animal and vegetable 

 substances as they were liable to encounter. It having been ascertained that the shape of the 

 glabella and axial lobe furnishes an indication of the degree of development of the alimen- 

 tary canal it is possible to infer something of the kind of food used by various trilobites. 



The narrow glabellas and axial lobes of the oldest trilobites would seem to indicate a 

 carnivorous habit, while the swollen glabellse and wider lobes of later ones probably denote an 

 adaptation to a mixed or even a vegetable diet. This can not be relied upon too strictly, 

 of course, for the swollen glabellse of such genera as Deiphon or Sphccrexochus may be due 

 merely to the shortening up of the cephalon. 



Walcott (1918, p. 125) suggests that the trilobites lived largely upon worms and con- 

 ceives of them as working down into the mud and prowling around in it in search of such 

 prey. While there can be no doubt that many trilobites had the power of burying them- 

 selves in loose sand or mud, a common habit with modern crustaceans, most of them were 

 of a very awkward shape for habitual burrowers, and how an annelid could be successfully 

 pursued through such a medium by an animal of this sort is difficult to understand. In 

 fact, the presence of the large hypostoma and the position of the mouth were the great 

 handicaps of the trilobite as a procurer of live animal food, and coupled with the rela- 

 tively slow means of locomotion, almost compel the conclusion that errant animals of any 

 size were fairly safe from it. This restricts the range of animal food to small inactive 

 creatures and the remains of such larger forms as died from natural causes. The modern 

 Crustacea are effective scavengers, and it is probable that their early Palaeozoic ancestors 

 were equally so. It is a common saying that in the present stressful stage of the world's 

 history, very few wild animals die a natural death. In Cambrian times, competition for 



