PART II. 



STRUCTURE AND HABITS OF TRILOBITES. 

 INTERNAL ORGANS AND MUSCLES. 



Granting that the trilobite is a simple, generalized, ancient crustacean, it appears justifi- 

 able to attribute to it such internal organs as seem, from a study of comparative anatomy, 

 to be primitive. 



The alimentary canal would be expected to be straight and simple, curving downward 

 to the mouth, and should be composed of three portions, stomodaeum, mesenteron, and proc- 

 todseum, the first and last with chitinous lining. In modern Crustacea, muscle-bands run from 

 the gut to part of the adjacent body wall, so that scars of attachment of these muscles 

 may be sought. At the anterior end of the stomodseum, they are usually especially strong. 

 From the mesenteron there might be pouch-like or tubular outgrowths. 



The heart would probably be long and tubular, with a pair of ostia for each somite. 



In modern Crustacea, the chief organs of renal excretion are two pairs of glands in the 

 head, one lying at the base of the antennae and one at the base of the maxillae. Only one 

 pair is functional at a time, but these are supposed to be survivors of a series of segmen- 

 tally arranged organs, so that there might be a pair to each somite of a trilobite. 



The nervous system might be expected to consist of a supracesophageal "brain," com- 

 prising at least two pairs of ganglionic centers, and a double ventral chain of ganglia with 

 a ladder-like arrangement. 



Besides these organs, a variety of glands of special function might be predicted. 



Reproductive organs probably should occur in pairs, and more than one pair is to be 

 expected. There is little to indicate the probable location of the genital openings, but they 

 may have been located all along the body back of the cephalon. 



It may be profitable to summarize present knowledge of such traces of these organs 

 as have been found in the fossils, if only to point out what should be sought. 



ALIMENTARY CANAL. 



Beyrich (1846, p. 30) first called attention to the alimentary canal of a trilobite, (Cryp- 

 tolithus goldfussi,) and Barrande (1852, p. 229) confirmed his observations. A number 

 of specimens of this species have been found which show a straight cylindrical tube or 

 its filling, extending from the glabella back nearly to the posterior end of the pygidium. It 

 lies directly under the median line of the axial lobe, and less than its own diameter beneath 

 the dorsal test. At the anterior end it apparently enlarges to occupy the greater part of the 

 space between the glabella and the hypostoma, but was said by the early observers to extend 

 only a little over halfway to the front. Beyrich thought the position of the median tubercle 

 indicated the location of the anterior end. 



Walcott (1881, p. 200) stated that in his experience in cutting sections of trilobites it 

 was a very rare occurrence to find traces of the alimentary canal. The visceral cavity was 

 usually filled with crystalline calcite and all vestiges of organs obliterated. There were, 

 however, some slices which showed a dark spot under the axial lobe, which probably rep- 

 resented the canal. In his restoration he showed it as of practically uniform diameter 

 throughout, and extending but slightly in front of the mouth. 



