Eyes of MoUuscs :ind Arthropods. 565 



present, which carried the development of these organs beyond tlie le- 

 quirements of the animai. The development was so rapid, and the iu- 

 ertia — if we may use the expression — so great, that a state of iu- 

 equilibrium was attaiued, now returning to a more stable condition by 

 a reduetion in the number of eyes. This furnishes us with a remarkable 

 instauce of reversed degeneration, in which the degradation begins, not 

 with the loss of the most reeently acquired characters, that is, the re- 

 duetion of the ommatidia to a simpler condition — but by the Isola- 

 tion of the perfected ommatidia; therefore retracing that developmental 

 Step which was the first to give rise to the eyes. 



The faceted eyes probably arise as modifications of the invaginated 

 ones : during this process, the latter become shallower, and the nervous 

 network reduced entirely to the rods of the retinophorae, while the re- 

 tinulae, whose rods disappear, are used solely to protect the retinidia 

 of the retinophorae from the lateral rays of light. 



Pectuuculus. 



The thick mantle edge of Pectmiculus contains an enormous num- 

 ber of gland cells, which scerete a quantity of thick slime, materially 

 adding to the difficulty of studying the eyes of the living animai. Al- 

 though the edge of the mantle is thus enlarged, the three folds, which 

 are present here as in Area and Pecten, are but slightly developed. The 

 inner wall of the Ophthalmie fold is much extended and constitutes the 

 greater part of the pigmented portion of the mantle. It is studded with 

 numerous oval, or round, madder browu, pigmentspots, which, in mauy 

 cases, are sharply defined areas of nearly black pigment that might 

 easily be taken for eyes ; in other instances, they are more irregulär and 

 lighter colored, without the sharp boundaries of the former. With the 

 esception of these pigment spots, the hypodermis of the Ophthalmie fold 

 is nearly colorless , or of a faint, yellowish white tinge, intensified at 

 the base of the velum to form a distinct band of brownish pigment, 

 parallel with the edge of the mantle. The velum is reduced to a color- 

 less, and inconspicuous ridge thrown into many irregulär folds. At the 

 base of the ridge , on the proximal side , is a distinct band of brown 

 pigment, in the median portion of the mantle, broken up into wedge- 

 shaped patches, each one consisting of radiating, pigmented lines. At 

 the anterior end of the mantle, the pigment is darker and forms a con- 

 tinuous, but narrow band. The cuticula is slightly thickened over the 



