INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 947 



The " spherical organ" is regarded by the author as being the 

 homologue of the shell-gland of the Mollusca ; both are local invagina- 

 tions, and while one gives rise to the shell the other forms the blasto- 

 derm cuticle. It has also a relation to the formation of the ectoderm 

 and mesoderm, for at the time when the first signs of the extremities 

 become apparent, and the spherical organ has taken up a definite posi- 

 tion, tlie yolk, lying below this last, begins to break up into spheres, 

 and this change gradually extends over the mass. It would seem 

 probable that these " Ballen " have their origin in the spherical organ, 

 and it may be that the cells of the ectoderm arise from the base of the 

 invagination. 



Structure of the Eye of Limulus.* — Dr. A. S. Packard, jun., 

 writes : — 



The eyes of the horse-shoe or king crab are four in number, con- 

 sisting of a pair of compound eyes situated on the side of the head, 

 and a pair of small, sim2)le eyes on the front of the head. As described 

 by A. Milne-Edwards and Owen, the optic nerves to these eyes are 

 very long, and close to each eye subdivide into an irregular plexus of 

 fine nerves, a branch being distributed to each facet composing the 

 compound eye. The structure of the eye is very unlike that of any 

 other Arthropod eye. The cornea is simjily a smooth convex portion 

 of the integument, which is much thinner than the adjoining part of 

 the chitinous skin. There are no facets, the cornea externally being 

 structureless, simply laminated like the rest of the integument. On 

 the internal side of the cornea are a series of solid chitinous conical 

 bodies, separated from one another by a slight interspace, and in form 

 resembling so many Minie-rifle balls. The conical ends of these solid 

 cones project free into the interior of the body, and are enveloped in 

 a dense layer of black pigment. Within the base of these cones are 

 secondary, shallow, cup-like bodies, or sliallow secondary cones. It 

 is these primary cones which, seen through the smooth, convex, trans- 

 lucent cornea, give the appearance of a faceted surface to the external 

 eye. 



All the parts thus far described, except the pigment layer, are 

 moulded with the rest of the crust ; and the large, long, slender cones 

 can be easily seen by viewing a piece of the cast-off eye, the solid 

 cones being seen projecting from the inner surface of the cast-off 

 cornea. 



The internal structure of the eye is very simple. There are no 

 cones ami no rods, but a branch of the optic nerve impinges directly 

 upon the end of the solid chitinous cone, as determined by removing 

 the layer of pigment with dilute jKjtash, and treating the section with 

 acetic acid, and then stiiiiiing witli picrocannine. So far as the 

 uutlior can ascertain, no Artliropod eye is so simple as that of Litnnlus. 



Tho observations wore based on a study of the lobstoi-'s eye from 

 preparations of very great beauty and delicacy, made for Iiini by Mr. 

 N. N. Mason, of Pi'ovidcnco, who has also made beautiful sections of 

 the Limulua oyc, after treating them in various ways. Tho question 



♦ ' Am. Natnriil.,' xiv. (1880) p. 212. 



