1884,] NATURAL SCIENCES OF PHILADELPHIA. 301 



could have existed, for the homologies of many organs, and their 

 intermediate states, show that wonderful metamorphoses in 

 function are at least possible." ' 



It will be my endeavor to sliow the stages of development of 

 the eye from the simple deposit of pigment in an epithelial cell 

 to the highest form known to us, that of the vertebrata. 



Invagination seems to be the most simple, as well as one of the 

 commonest, methods by which organs are formed in the animal 

 series. The formation of the gastrula, of the medullary canal, the 

 development of glands, etc., etc., by invagination, are cases too 

 well-known to require further comment. The formation of the 

 eye, ear, and nose, form no exception to this rule. 



In a previous paper ^ I have endeavored to show that the simplest 

 expression of an organ of sight is found in the Lamellibran- 

 chiata. These simple organs, however, are not morphologically 

 the primitive visual organs of the group, but adaptive organs, 

 the ancestral eyes being present, in a few forms, only for a short 

 time during the free larval stage of the animal ; it is lost when 

 the animal becomes fixed and the head excluded from the light. 



We will hastil}' review these simple eyes. 

 One of the simplest cases is found in Ostrea 

 virginica (fig. 1), in which we have, on the free 

 edge of the mantle, a number of epithelial cells fig. i.— visual ceiis 



... , / \ T -1 j> • y of Ostrea virginica. c, 



containing a nucleus (n), a deposit of pigment cuticle; p, pigment; 

 (p) in their exterior extremities, and on the "''^"°^"^' 

 outer surface a fine transparent, refractive cuticula (c). There 

 seems to be no protection for the organ, save the power of with- 

 drawal of the whole mantle within the valves of the shell. 

 Experiment conclusively proves that sight exists in these 

 animals, as shown by Ryder.^ 



We next find these pigmented visual organs confined to a 

 certain point of the mantle which has become specialized into 

 the so-called siphon. In Venus mercenaria we have these cells, 

 unprotected on the external surface of the siphon, but at the 

 same time some cells are more or less protected at the base of 

 the tentacles ; but as this animal is able to retract the entire 



^ Darwin, Chas., Orig, of Species, p. 182. 



- Sharp, B., On the Visual Organs in Lamellibranchiata. Mittheil. a. d. 

 Zool. Stat, zu Neapel, Bd. v, 1884, p. 447. 



^ Ryder, J. A., Primitive Visual Organs. Science, vol. ii. No. 44, 1883, 

 ]). 739. 



