NATURAIv HISTORY OF AMERICAN LOBSTER. 233 



lobate, and finally stalked. In the first larva the stalks are immobile but very large, 

 being relatively four times longer than in the adult. From the fourth stage the faceted 

 eye is typically borne at the apex of a cylindrical movable stalk, which projects from 

 either side of the base of the rostrum. Each stalk (fig. i, pi. xxxv) is capped with a 

 hemispherical surface, over which the cuticle has become modified into a thin flexible 

 membrane as transparent as glass. Through it is seen the black pigment which defines 

 the retinal area. This window-like cornea is interrupted by a process which juts in 

 like a peninsula from the opaque shell at a point where the field of vision seems to be 

 interrupted by the rostrum. 



After the first larval stage the eyestalks recede somewhat until the lobster attains 

 a length of from i^ to 3K inches, when their prominence is again very marked. In 

 short, they now assume the form and relative size of certain fossil Crustacea from which 

 the modem lobsters have probably descended. 



The structure of the compound eye of the crustacean appears to be extremely 

 complicated, because it is composed of units repeated many thousands of times. As 

 was shown in 1889," it is wholly derived by difi'erential growth from a single plate of 

 columnar ectodermic cells, the optic disk, which arises very early in development on 

 either side in front of the future mouth and before the buds of the antennules are formed. 



When the lobster's eye is examined with a hand lens, its clear corneal membrane 

 has the appearance of a glass mosaic, composed of minute square disks of great uniformity 

 both in size and arrangement, especially in its central parts (fig. 2 and 3, pi. xxxv). 

 Each disk is the f acet of a n eyelet ot ommatidium of the compound eye, and each sup- 

 plies a part of the mosaic image produced in vision when the Ught is sufiiciently strong. 

 Each eyelet is developed from a cell cluster of the optic disk and this in turn from a 

 single columnar cell of the primary optic plate. 



The axial part of the ommatidium consists of (i) the corneal lens secreted by 2 

 ijnderlying.i:sll5, (2) the refractive cone derived from 4 cone cells, and (3) a long striated 

 and sensitive rod, the rhabdom, secreted and sheathed by 7 retinular cells, in addition 

 to 2 peripheral pigment cells which surround the crystaUine cone; in this rod also a 

 nerve fiber terminates at the level of a basement membrane which divides the proper 

 eye from the complex optic ganglia, muscles, and other tissues contained in the rest of 

 the stalk. In ordinary dayUght each eyelet is completely isolated by its sheath of 

 black pigment cells, all of which display ameboid movement, but which respond dif- 

 ferently to the intensity of the light stimulus. 



In 1890, while working at the laboratory of the U. S. Fish Conunission at Woods 

 Hole, Mass., I showed by experiments upon the prawn Palamonetes vulgaris that when 

 this animal was placed in total darkness there was an immediate adjustment of the 

 pigment cells of the ommatidium, in consequence of which the whole eye became intensely 

 black and prominent, and that when returned to the light the eye began to lighten in 

 a few minutes and in a relatively short time assumed its normal daylight appearance. 

 It was shown that the blackening was due to a forward movement of processes of the 



oThe developmCQt o£ the compound eye of Alpheus. Zoologischer Anzeiger, bd. xn, p. 164-169, fig. 1-5. I^ipzig, 1889. 



