STRUCTURE AND AFFINITIES OF TRILOBITES. 



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Edriophthalma, Leach. 



Animals having their eyes ^ 

 sessile. 



Aberrant Group. Orders. 



Amphipoda, Lat. Head distinct with four antennse. Feet 



thick and crustaceous. Animals not un- 

 dergoing metamorphosis. 

 Trilobita, Brongn. Head distinct without antennse. Feet rudi- 

 mentary, soft, and membranaceous. 

 Entomostraca, Lat. Head rarely, if ever, distinct from thorax, 



but provided with antennse. Feet always 

 distinct. Animals undergoing metamor- 

 phosis. 



With regard to the habits of true Trilobites, these animals have been supposed by sonie natu- 

 ralists to be parasitical 3 but I conceive this hypothesis not to be very tenable, since almost all ex- 

 isting articulated parasites that adhere externally to other animals have strong feet, hooked at the 

 end for that purpose. Now, the Trilobites certainly had no such strong crustaceous hooks to their 

 feet, or these hooks would have long since been detected. The close affinity of Trilobites to Bo- 

 pyrus does not prove a parasitical mode of life, for Sphceroma and other Cymothoadce which, like 

 Trilobites, have the power of coiling themselves up into a ball, are not parasitical, although so 

 close in affinity to the parasitical genus Cymothoa. Nay, it has been said that the Cymothoadce 

 and Epicarides do not draw their nourishment directly from the animals to which they adhere ; but, 

 on the contrary, live entirely on the animalculse brought to them in the water by the play of the 

 branchise, near which they always take their post. Still the close connexion of Trilobites with 

 Bopyrus, and their feet almost null, if not entirely so, induce me to think that these animals must 

 have been to a certain degree sedentary. The flat under surface of their bodies, and the lateral 

 coriaceous margin of several species, which is so analogous to that of Chiton, make it probable 

 that they adhered with a soft articulated underside either to rocks or fuci. They appear to have 

 been among Crustacea what the Vermes or white-blooded worms are among Ametahola, — often 

 without eyes, and always without antennse or distinct feet. If they had feet, as Audouin and Goldfuss 

 imagine, and, as indeed is most probable, they must have been so small, so membranaceous, so soft, 

 and so rudimentary, as almost to be useless to the animals for locomotion. The mouth, so analo- 

 gous to that of Apus ) makes us imagine that the Trilobites were carnivorous 3 and they may possibly 

 have fed on Acrita, Annelida, or naked Mollusca. That they had to search for their food, and 

 that they possessed some small power of locomotion, is to be inferred from their highly organized 

 eyes ; for no truly sessile animal is provided with sight. The Balanus, when it becomes sedentary, 

 loses its eyes, as does also, in like case, the female Coccus. I imagine, therefore, that although the 

 Trilobites were to a certain degree sedentary, more particularly the blind ones, they must have had 

 some power of crawling over a flat surface ; but whether they moved by rudimentary, soft, mem- 

 branaceous feet, or whether it was by means of the undulation of setigerous segments, like the 

 earthworm, or by wrinkling the under surface of the abdomen like a Chiton, are questions yet to 

 be determined. One thing, moreover, is in my opinion clear, from their longitudinally trilobed 

 form and lateral coriaceous margin, namely, that they had the power of adhering to a flat surface 

 like a Chiton, Bopyrus, or Coccus. While thus sedentary, the hard, although thin, dorsal shell 

 probably saved them in some degree from the attacks of fishes, just as that of Chiton protects such 

 Mollusca from all fishes except the Scaridce. The Trilobites probably (like Ostrece, Chitones, 

 Cocci, and other sedentary animals), adhered in masses one upon the other, and thus formed those 

 conglomerations of individuals which are so remarkable in certain rocks. 



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