ECONOMY OF THE DAPHNIA. 16D 



ated, and from which it derives its origin. Both these exceptions 

 to the general rule are, however, I am convinced, apparent rather 

 than real. I have repeatedly observed that the organ is double in 

 the earliest stages of embryo Hfe, the two pigmentary masses being 

 perfectly distinct (see Figs. i6 and 27), and even in adults, if a 

 good front view be obtained, the duplicature is still indicated by a 

 notch ; Leydig, too, I find, has observed such to be the case. 

 This coalescence of the eyes seems to be in conformity with that 

 general alteration which takes place in the body of the embryo, 

 whereby it is reduced from a depressed larval form to a 

 compressed condition in the adult. The remaining anomaly 

 implied in the immersion of the eye within the body cavity, 

 puzzled me for a long time, and was only revealed lately by the 

 accidental circumstance of my having under examination a 

 specimen deeply coloured by the red tinge which these animals 

 sometimes exhibit, and which resides in the plasma or fluid of the 

 blood. The eye is, as in all other cases, formed by an invagination 

 of the epiblast, or external cellular tunic of the embryo ; but 

 the invagination in this case, I believe, proceeded farther than 

 usual, so as to suffice not only for the formation of the organ, 

 but for its reception in a cavity between it and the cuticle, or 

 hard outer investment. This cavity becomes subsequently closed 

 at the neck by the ingrowth of the epiblast, and forms an internal 

 sac {0 s, Fig. 11), in which the eye rests, and is balanced therein 

 by muscular action. The sac is, I have reason to believe, filled 

 with nothing but water. The cuticular covering, which in other 

 Crustacea and in insects takes part in the formation of the corneal 

 lenses, does not enter into the composition of the eye of Daphnia; 

 no such lenses exist, a deficiency which necessarily follows the 

 dissociation of the organ from its internal surface, and its mobile 

 condition. It consists therefore of only the crystalline cones, and 

 the rods surrounded by pigment cells ; which form the epidermic 

 structures in other arthropods, arranged in a nearly spherical 

 form. In the embryo the eye is stationary, the sac not having yet 

 been formed. It is, moreover, in close proximity with the optic 

 ganglion, another point of rapprochement with the more typical 

 forms, which, as development proceeds, disappears by the formation 

 between them of a number of nervous cords, one apparently for 

 each visual rod, thus providing for the subsequent characteristic 

 mobility of the organ. 



In the adult the optic sac is rounded in front, where it is in 

 close proximity to the general epidermic layer underlying the 

 cuticle, and its under surface towards the optic nerve is of a 

 bulging form, somewhat Uke the surface of a cushion puckered in 



