18 A. S. PACKAKD, JR., ON THE ANATOMY 



Crustacea, and the green glands of the Decapoda. It should be remembered, 

 nevertheless, how difficult it is to find the excretory duct of the green gland, though 

 its inlet is very apparent. It is probable that we have here to deal with a new form 

 of kidney, adding a fourth kind to the three forms of renal organs existing in the 

 Crustacea.^ 



Structure of the Eyes of Limulus. 



After we had made some researches on the structure of the compound eyes of Limulus, 

 and had ascertained that their structure is quite unlike that of other Arthropodous eyes, 

 having a chitinous lens and no rods and cones, we had the opportunity of examining 

 Grenacher's elaborate work entitled Untersuchungen uber das Sehorgan der Arthropoden, 

 insbesonderer der Spinnen, Insecten und Crustaceen? 



We have little to add to Grenacher's account of the histology of the compound eye, 

 and our studies confirm the accuracy of his account and the three drawings he publishes 

 of the structure of the retinula and the rhabdom, although we have failed to find the layer 

 of epithelial cells extending up between the corneal lenses and next to the pigment layer ; 

 these are much less regular in their arrangement than drawn by Grenacher, and seem to 

 be simply connective tissue cells, which are as abundant away from the lenses as next to 

 them. We have also not succeeded in observing that the optic nerve fibres end in the 

 manner indicated in his drawing. We may here say that we had examined sections of the 

 compound eye of Limulus, and had made out the leading points in its structure before 

 seeing Grenacher's work. 



The following account is based upon observations made upon sections cut for us by Mr. 

 Mason. They are taken in most cases from living specimens, placed in alcohol, and 

 hardened in gum arable ; and either stained with picro-carmine, or else the pigment layer 

 dissolved wholly or in part with nitric acid in order to uncover the ends of the corneal 

 lenses and to show the structure of the retinula and rhabdom. The subdivisions of the 

 optic nerve were best showed in slices stained with picro-carmine, the nervous substance 

 being but partially colored and contrasting well with the highly tinged connective tissue 

 by which the nerves are surrounded. In order to study the eye of Limulus intelligently 

 Mr. Mason kindly made for us numerous exquisite sections of the eye of the lobster.^ 



Plate 6, fig. 1, represents an actual section of the eye, with its exterior convex surface, 

 its lenses, retina and nerves. The surface of the eye is convex, smooth and polished. 

 The integument over the eye suddenly diminishes in thickness to form the cornea ; it is 

 solid and chitinous as in the rest of the integument, and is composed of three layers : 

 the outer and thinner more solid one (Ij), which is clear yellow or amber-colored; the 

 middle {\), which is duller yellow and is finely laminated and softer, being partially 



1 SeeEug. Wa8siliew. Ueber die Nieren des Flusskrobses. the optic nerve-fibres, so that the structure of the eye could 



Zoologischer Anzeiger, p. 221, 1878. readily be studied. AVe did not perceive that the anatomy 



!" Von H. Grenacher. Gbttingen, 1879, 11 lith. taf. 40, of the eye of /iomm-us americajms diiiered in any important 



pp. 188. respect from that of the European lobster as worked out by 



8 These sections made by Mr. Mason were remarkably Mr. Edwin T. Newton. Quart. Journ. of Microscopical 



successful, the slices being thin enough to include a layer of Science, 1873, p. 339. 

 hundreds of facets and rods and cones but one deep, with 



