Eyes of Molluscs and Arthropods. 729 



for the retiniüae, or pig'mented cells to lose their rods , and become 

 transformed into g'aiig'lionic ones. According* as tbis process is more or 

 less complete, we bave a greater or less teudeney to form layers of re- 

 ti uiilae, ganglionic cells, and retinophorae. 



It is diffieiüt to g-ive any satisfactory definition, covering all ckses, 

 of a single or double layer of cells. Compound layers may obviously 

 arise in two way^- by folding, er by delamination. He who would re- 

 gard the retina of Cephalopods , Heteropods, and Pecten^ for instance, 

 as a single layer, must do the same for the retina of the Vertebrates 

 (not including the ehoroidea). If we regard the retina of Vertebrates 

 as single layered, because it originated from a single embryonic layer, 

 then we would bave the same right to say that the brain consists of a 

 single layer of cells. We would also bave to regard the entire omma- 

 teum of the compound Arthropod eye, together with its underlying optic 

 ganglion, as forming a single layer, since both bave originated from a 

 single row of hypodermic cells. But both of these extremes lead to an 

 evident absurdity. The retina of the Cephalopods or Heteropods is 

 no more formed of a single layer of cells than the optic ganglion and 

 ommateum of the Arthropod eyes; the latter represent an extreme con- 

 dition of the same kind of change that is going on in the eyes of 

 Cephalopods, Pecten, or Vertebrates. In ali sensitive layers, a dela- 

 mination of the single row of embryonic, hypodermic cells, into sense 

 and ganglionic ones, takes place. In Cephalopods for instance, the 

 delamination has resulted in the production of the optic ganglion and 

 the retina, both of which are never entirely separated from each other. 

 The retina then undergoes stili further delamination, in that some of 

 its cells become ganglionic, but without being separated any distance 

 freni the true sense cells. The retina in Vertebrates has undergone an 

 exactly similar process. The Separation of the retina from the optic 

 ganglion (the thalamencèphalon) takes place very early, and is modified 

 by secondary changes. A further delamination of the primitive retina 

 into several layers gives rise to numerous strata of ganglionic cells, 

 which only differ from those of the brain in that they are not so far re- 

 moved from the true sense cells. The same process has occurred in 

 Pecteìi, as well as in Haliotis. except that in the latter case the number 

 of ganglionic cells closely united to sensitive ones is very small. In 

 Arthropods, the delamination of the single row of primitive, hypoder- 

 mic cells early gives rise to the optic ganglion and ommateum, ali the 

 ganglionic cells being removed from the ommateum to the optic gang- 

 lion. An ommateum therefore consists entirely of sense cells and pro- 



