AND EMBRYOLOGY OP LIMULUS. 19 



stained reddish by carmine ; and a third (I3) thicker layer, less laminated and pierced by 

 nutritive canals (p), filled with connective tissue and directly communicating with the 

 body cavity. From the cornea project obliquely inwards large, long, solid, conical 

 processes (cl). These are the " corneal lenses " of Grenacher, which he regards as homolo- 

 gous with the corneal lenses of larval insects and of Arachnida. We see no reason to dissent 

 from this opinion. These corneal lenses are long, cylindrical, obtusely pointed, sometimes 

 quite sharp, at the end. They point inwards more or less obliquely towards the centre of 

 the eye. Those (as at fig. 2a) near the periphery of the eye are longer and slenderer and 

 more oblique than those in the centre, the latter being considerably shorter and blunter 

 (plate 6, fig. 2). These lenses are developed from the third, a portion of the second or 

 more laminated layer of the cornea filling up a conical space (fig. 2a, h,) at the base of 

 the cone ; the laminae composing this shallow cone within the larger cone are continuous 

 with the laminated layer of the cornea, and like it are stained reddish by the carmine, 

 while the cone itself remains unstained, of a clear amber color, and is structureless ; 

 sometimes one or two curved lines being seen parallel to the periphery of the end of the 

 cone. That the corneal lens is solid is proved not only by its appearance, as seen 

 in numerous sections, but by the frequent marks of the razor, and by the laminated 

 structure of the inner conical portion. What relation, if any, the conical part {h) has 

 physiologically to the corneal lens, we are not prepared to state. 



The terminal half, or sometimes third, of the corneal lens is enveloped in the pigment 

 layer or retina, (plate 6, fig. 3, rt), which is morphologically a continuation or modification 

 of the dark hypodermis {hy). The layer is continuous between the ends of the solid corneal 

 lenses, but is produced at the ends of the latter into cones of corresponding size [rtc), 

 which project into the body-cavity, and are enveloped by the dense connective tissue. 

 As stated by Grenacher, this pigment layer is composed of modified epithelial cells, which 

 are very long and slender, with a minute nucleus (fig. 3, rcZ). It is very difl&cult to make 

 out these cells, and we should have overlooked them had not Grenacher described and 

 figured them ; finally, however, we could trace them, in preparations treated with acid, 

 into the hypodermis, where the cells are also long and slender, though shorter than in the 

 retina. Plate 6, fig. 3, rcl, represents these retinal cells, as seen at the end of an acute 

 corneal lens, and their relation to the rhabdom (rhab). 



Besides the retina, the soft parts of the entire compound eye of Limulus consists of 

 a large mass of connective tissue {ct), lying under and next to the retina and finely gran- 

 ular, permeated by the irregular tortuous branches of the optic nerve. The cells and 

 granules of this specialized subocular portion of the connective tissue forming the paren- 

 chyma of the cephalothorax are smaller than elsewhere ; they are nucleated, and the 

 tissue stains paler crimson by the picro-carmine, than the connective tissue beyond the 

 subocular area, which remains darker brown, with coarser granules. The arteries, 

 ovarian-tubes and liver-tubes, rarely penetrate into the subocular area ; and the branches 

 of the optic nerve do not wander into the region beyond. Fig. 1, ar, represents the cut 

 ends of two minute arterial branches, ov represents the cell-eggs of the end of an ovarian 

 tube, and I indicates the much larger sections of a liver-tube ; these vessels constitute the 

 greater part of the soft portions of the cephalothorax, being brown or yellowish brown, 

 and enveloped in a dense connective tissue. 



