HEART. . 87 



V 



8. The median eye is borne on a tubercle or mound in the Ordovician and Silurian trilobites, while the 

 tubercle is rarely noticed in the Devonian and in few Cambrian forms. In the Devonian forms, similarly 

 as in many crustaceans and in later growth-stages of some asaphids, the strong development of the lateral 

 eyes may have led to a loss of the parietal eyes. In the Cambrian genera evidence is present to suggest that 

 the parietal eyes consisted only of transparent spots or lens-like thickenings of the exoskeleton, hardly 

 noticeable from the outside. 



9. It is a priori to be inferred that the trilobites should, as primitive crustaceans, have possessed median 

 or parietal eyes. 



As a student, I accepted Professor Beecher's dictum that this tubercle represented a 

 median ocellus, but more recently a number of things have led me to the view that it is 

 the point of attachment of the ligament by which the heart is supported. 



The chief arguments against its interpretation as a parietal eye seem to be that its 

 structure is not absolute proof, being capable of other explanation; its position is variable, 

 in front, between, or back of the eyes; it is exactly like other tubercles on the median line, 

 especially the nuchal spine or tubercle, and the similar ones along the axial lobe of the 

 thorax; and it is not present in the protaspis or very young trilobites. 



1. The structure disclose'd by Ruedemann's sections, a sort of sac-like cavity beneath a 

 thinned test, can be explained as a gland, a ligamentary attachment, or a vestigial spine, as 

 well as an eye. In a section of Asaphus expansus, which I made some years ago when try- 

 ing to get some light on this problem, there is a similar cavity under the pustule, but a 

 secondary layer of shell lay beneath it and apparently cut it off from the glabellar region, 

 thus indicating that it had lost its function in the adult of this animal. Sections through 

 the tubercles of the glabella of Ceraurus show all of them hollow, with very thin upper 

 covering or none at all, and their structure is not unlike that of the tubercle of Cryptolithus. 

 In fact, sections can be seen in Doctor Walcott's slices which are practically identical with 

 the one Ruedemann obtained from Cryptolithus. Since it is obvious that not all of the 

 pustules of a Ceraurus could have been eyes, the evidence from structure is rather against 

 than for the interpretation of the median pustule as such an organ. 



2. The position of the tubercle varies greatly in different genera. Where furthest for- 

 ward (Tretaspis, Goldius), it is just back of the frontal lobe, while in some species of asa- 

 phids it is in the neck furrow. In species with compound eyes it is frequently between the 

 eyes, but more often back of them. If its history be traced in a single family, it is gen- 

 erally found farthest forward in the more ancient species and moves backward in the more 

 recent ones. The eyes do this same thing, but the median tubercle goes back further than 

 the eyes. This can be seen, for example, in the American Asaphidre, where the pustule is 

 up between the eyes of Hemigyraspis and Symphysurus of the Beekmantown and back of the 

 eyes of the Isotclus of the Trenton. Turning now to the under side of the head, it appears 

 that the tubercle bears a rather definite relation to the hypostoma. If the hypostoma is short, 

 the tubercle is well forward. If long, it is far back on the head. It seems in many cases 

 to be just back of the posterior tip of the hypostoma, or just behind the position of the 

 mouth, while in others it is not as far back as the tip of the hypostoma. 



The median tubercle is in many cases developed into a long spine. This is usually in 

 an ancient member of a tubercle-bearing family, and suggests that in most cases the tubercle 

 is a vestigial organ. An example of this occurs in Trinude aides, the most ancient 

 of the Trinucleida:. Trinudeoides reussi (Barrande) (Supplement, 1872, pi. 5, figs. 17, 18) 

 has a very long slender spine in this position. It could be explained as an elevated median 

 eye, but it also very strongly suggests the zoasal spine of modern brachyuran Crustacea. 



