98 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



The holochroal eyes are of two kinds, one with plano-convex and one with biconvex 

 lenses. The latter would seem to be mechanically the more perfect of the two, and it is 

 worthy of note that the trilobites possessing the biconvex lenses have, in general, much smaller 

 eyes than those with the other type. 



If, as some investigators claim, the parietal eye of Crustacea originates by the fusion of 

 two lateral ocelli, trilobites show a primitive condition in lacking this eye, which may have 

 originated through the migration toward the median line of ocelli like those of the Trinu- 

 cleidoe. 



SEX. 



That the sexes were separate in the Trilobita there can be very little doubt, but the 

 study of the appendages has as yet revealed nothing in the way of sexual differences. One 

 of the most important points still to be determined is the location of the genital openings. 



In many modern Crustacea, the antennae or antennules are modified as claspers, and it 

 is barely possible that the curious double curvature of the antennules of Triarthrus indi- 

 cates a function of this sort. The antennules of many specimens have the rather formal 

 double curvature, turning inward at the outer ends, and retain this position of the frontal 

 appendages, no matter what may be the condition of those on the body. Other specimens 

 have the antennules variously displaced, indicating that they are quite flexible. It is conceiv- 

 able that the individuals with rigid antennules are males, the others females. 



It is interesting to note that the antennules of Ptychoparia pcrmulta Walcott (1918, pi. 

 21, fig. i) have the same recurved form. All the specimens of Neolenus, however, show very 

 flexible antennas. 



Barrande and Salter laid great stress upon the "forme longue" and "forme large" 

 as indicating male and female. This was based upon the supposition that the female of 

 any animal would probably have a broader test than the male, a hypothesis which seems to 

 be very little supported by fact. In practical application it was found that the apparent dif- 

 ference was so often due to the state of preservation or the confusion of two or more 

 species, that for many years little reference has been made to this supposed sex difference. 



EGGS. 



In his classic work on the trilobites of Bohemia, Barrande described three kinds of spheri- 

 cal and one of capsule-shaped bodies which he considered to be the eggs of trilobites. After 

 a review of the literature and a study of specimens in the collections of the Museum of 

 Comparative Zoology, it can be said that none of these fossils has proved to be a trilobite 

 egg, but that they may be plants. A full account of them will be published elsewhere. 



Walcott (1881) and Billings (1870) have described similar bodies within the tests of 

 Calymenc and Ceraurus, but without showing positive evidence as to their nature. 



METHODS OF LIFE. 



This is a subject upon which much can be inferred, but little proved. Without trying 

 to cover all possibilities, it may be profitable to see what can be deduced from what is known 

 of the structure of the external test, the internal anatomy, and the appendages. This can, 

 to a certain extent, be controlled by what is inferred from the strata in which the specimens 

 are found, the state of preservation, and the associated animals. (For other details, see 

 the discussion of "Function of the Appendages" in Part I.) 



