VARIOUS GLANDS. 89 



it may perhaps be justifiable to point out that in some of the Agnostidas there is a median 

 tubercle on both shields, and since it has not yet been demonstrated beyond question which 

 shield is the cephalon, to say which one is a parietal eye and which one is a tubercle is im- 

 possible. In other words, the parietal eye can not be differentiated from any other tubercle 

 except by its position. 



4. One of the as yet unexplained features of the protaspis of trilobites is the absence 

 of the "nauplius eye." Beecher (1897 B, p. 40) explained this on the ground of the 

 extremely small size of the protaspis and the imperfection of the preservation. If the me- 

 dian tubercle were really a median eye, it should be present in the protaspis and the earlier 

 stages of the ontogeny, even if not in the adult, and should certainly appear before the com- 

 pound eyes. (In Liiniilns, however, the compound eyes appear first.) The median eye has 

 not so far been seen in any young trilobite in any stage previous to that in which compound 

 eyes are present. The full ontogeny is not known of any species with compound eyes in 

 which the median tubercle is present in the adult, but theoretically the median eye should be 

 most prominent in the young of just those primitive trilobites about whose development most 

 is known. 



NERVOUS SYSTEM. 



There has been a rather general impression among students of trilobites that the eye- 

 lines, which should be differentiated from the genal caeca, denote the course of the optic 

 nerves, but no other evidence of the nervous system has been found, save the so called 

 nervures which have been discussed above. In Apus the nerves leading to the eyes come 

 off from the anterior ganglion or "brain" and run directly to the eyes. If conditions were 

 similar in the trilobites, the "brain" was beneath the anterior glabellar lobe, provided, of 

 course, that the eye-lines do indicate the course of the optic nerve. 



The ontogenetic history of the eye-lines of trilobites with compound eyes is instructive, 

 and has already been discussed by Lindstroem (1901, pp. 12-25), Dut ne did not cite the 

 case of Ptychoparia, which is particularly interesting, because in this genus both eye-lines 

 and "nervures" are present. Beecher (1895 C, p. 171, pi. 8, figs. 5-7) has shown that in 

 Ptychoparia kingi the eye-lines of a specimen in the metaprotaspis stage run forward at a 

 low angle with the glabella, while in the adult their course is nearly at right angles to it. They 

 have therefore swung through an arc of at least 60 and in so doing have had ample oppor- 

 tunity to become coincident with the primary trunks of the genal caeca. Once that was ac- 

 complished, it is quite likely that the one fold in the shell would continue to house both 

 structures. In other trilobites, there is a similar backward progression of the eye-lines. 



As would be expected, the ventral ganglia and the longitudinal cords left no trace in 

 the test. Since each segment has appendages, there was probably a continuous chain of 

 ganglia back to the posterior end of the pygidium. 



VARIOUS GLANDS. 



Dermal glands. The surface of many trilobites is "ornamented" with pustules and 

 spines which on sectioning are nearly always found to be hollow, and in many cases have a 

 fine opening at the tip. While it is generally believed that the purpose of these spines was 

 protective, yet it is possible that many of them were merely outgrowths which increased 

 the area through which the respiratory function could be carried on. It will be recalled 



