82 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



GASTRIC GLANDS. 



Taekel's suggestion, quoted above, that the so called "nervures" seen on the under sur- 

 faces of the heads of some trilobites are really glands for the secretion of digestive juices, 

 is at least worthy of consideration. Moberg, however (1902, p. 299), suggested that these 

 markings probably had something to do with the eyes rather than the stomach. He says in 

 part (translation) : 



In general we can now say that such features are common to all the eyeless Conocoryphidae. With the 

 conocoryphs I include Elyx and consider Harpides as at least closely related. Similar impressions are also 

 found in forms with eyes, as, for instance, in the Olenidse, but here such radiate partly from the border 

 of the eye, partly from the front end of the glabella, partly from the [visual surface of the] eye, and some- 

 times from the angle between the occipital ring and the glabella. They therefore go out from such different 

 points that they can not possibly be -branches of the liver. It would also be very remarkable if such an 

 important organ should have been developed in a few eyeless forms, but have failed to leave the least 

 trace in the rest of the trilobites. 



Lindstroem (1901, pp. 18, 19, 33; pi. 5. fi g s - 2 9. 3 1 ! P 1 - 6 , fi g s - 43-45) has discussed 

 these markings and given beautiful figures showing their appearance in Olcnus, Pambolina, 

 Efax, Conocoryphe, and S oleno pleura. He decided that they were to be explained as branches 

 of the circulatory system, comparing them with the veins and arteries of Limulus. He 

 pointed out that there was a coincidence between these markings and the position of the eyes, 

 and suggested a causal connection with the latter. 



Beecher (1895 B, p. 309), also from a comparison with Limulus, suggested that the 

 eye-lines of Cryptolithus, Harpes, Conocoryphe, Olenus, Ptychoparia, Arethusina, etc., prob- 

 ably represented the optic nerves, and since the eye-lines are usually the main trunks of the 

 dendritic markings, it is fair to assume that he considered the whole as due to branches of 

 nerves. 



Reed has recently (1916, pp. 122, 173) discussed -these lines as developed in the Tri- 

 nucleidse, and seems to accept Beecher's explanation. 



Three explanations of the "nervures" are thus current, and the authors of all of them 

 refer us to Limulus as proving their claims! So far as general appearance goes, the mark- 

 ings on the trilobites more closely resemble the veins of a Limulus than either the nerves or 

 "liver" of that animal. The veins, however, are not in contact with the dorsal shell, but 

 are buried in the liver and muscles, while the arrangment of the arteries, which are dorsal 

 in position, is quite unlike what is seen in the trilobites. 



The term nervures, as applied to these markings, is not only misleading, but an incor- 

 rect use of one of Barrande's words, for by nervures. he meant delicate surface markings. 

 Until the real function of the organs which made these markings is definitely established, it 

 may be well to call them genal cceca, for they obviously were open tunnels ending blindly, 

 whatever they contained. 



The question of the function of the genal caeca can not, in any case, be settled by an 

 appeal to Limulus, and it is doubtful if it can be settled at all at the present time. Cer- 

 tain things tend to show that Jacket's explanation is the most plausible, and these may be 

 briefly set forth. 



Walcott (1912 A, pp. 176, 179, pis. 27, 28) has described specimens of Naraoia and 

 Burgcssia in which similar markings are well shown, and where they are obviously con- 

 nected with the alimentary canal just at the anterior end of the mesenteron. In Burgessia, 

 which seems to be a notostracan branchiopod, the trunk sinuses are very wide, and the ap- 



