24 A. S. PACKARD, JR., ON THE ANATOMY 



In a section (transverse) of the cornea of Bathyurus longistrinosus, received from 

 Mr. Walcott, the lenses are seen to be very irregular, five- or six-sided, and very 

 irregularly grouped, not arranged in distinct rows. 



From the facts here presented, it would seem evident that the hard parts of the 

 eye of the Trilobites and of Limulus are, throughout, identical. The nature of the 

 soft parts will, as a matter of course, always remain problematical; unless the dark 

 line indicated in plate 6, fig. 9 {rt f) really represents the outer edge of the pigment 

 of the retina ; but however this may be, judging by the identity in structure of 

 the solid parts, we have, reasoning by analogy, good evidence that most probably 

 the eye of the Trilobites had a retinal mass like that of Limulus, and that the 

 numerous small branches of the long slender optic nerve (for such it must have 

 been) impinged on the ends of the corneal lenses. It has been shown by Grenacher 

 and myself that the eye of Limulus is constructed on a totally different plan from 

 that of other Arthropods ; I now feel authorized in claiming that the Trilobite's eye 

 was organized on the same plan as that of Limulus ; and thus when we add the 

 close resemblance in the larval forms, in the general anatomy of the body-segments, 

 and the fact demonstrated by Mr. Walcott that the Trilobites had jointed round 

 limbs (and probably membranous ones), we are led to believe that the two groups 

 of Merostomata and Trilobites are subdivisions or orders of one and the same sub- 

 class of Crustacea, for which we have previously proposed the term Palaeocarida. 



Structure of the simple eyes or ocelli. Owing to insufiicient material, Grenacher did not 

 study the simple eye of Limulus. The structure of an ocellus repeats very closely that of 

 one of the individual facets or members of the compound eye. At the point where a 

 simple eye is situated, i. e., on each side of the median spine near the front edge of the 

 carapace, the chitinous integument suddenly becomes much thinner ; the integument is 

 divided as in the cornea of the compound eye into three portions, an outer thin yellow clear 

 portion ; a much thicker finely laminated part with fine granules and capable of being 

 stained reddish ; and a much thicker clear part, which has about a dozen layers, not seen in 

 the third inner layer of the integument next to the edge of the compound eye. The 

 integument is also penetrated throughout by canals filled with connective tissue. The 

 surface of the cornea is slightly convex. Next to the base of the large corneal lens, there 

 is a chitinous portion {p) which is less dense than the adjoining clear part, becoming 

 stained a pale crimson by picro-carmine. Just as in the facets of the compound eye the 

 laminated part of the cornea extends conically into the base of the corneal lens, forming 

 a cone within the larger lens; this part (A) is less dense than the lens, and is usually 

 more distinctly conical than the lens itself. The latter is a large solid mass of chi- 

 tin, with two curved lines (plate 5, fig. 13, cl) in some examples, showing a slight ten- 

 dency to lamination ; in form it is longer than thick, and very obtusely rounded at the 

 end, being as thick near the end as at the base; in form therefore the lens differs 

 decidedly from that of the corneal lenses of the compound eye. That the corneal lenses 

 of both simple and compound eyes of Limulus are solid is proved by the fact that they 

 do not stain reddish like the laminated portion of the cornea and adjacent integument, 

 and also because they are excavated as solid cones projecting inwards ^from the cast 



