64 LAMELLIBRANCHIA. 



The invaginations, the optic nature of which is, indeed, doubtful, are pit- 

 like depressions of the epithelium, in the cells of which pigment is deposited, 

 while the pit itself becomes filled with a mass of what appears like a secretion 

 (conjectured to be a lens). 



The compound eyes arise as convex thickenings of the mantle-epithelium 

 at certain points. In these, conical sensory cells are distinguished from the 

 pigment-bearing and supporting cells lying between them by the development 

 of crystal cones and a cornea. The visual cells are connected with the fibres 

 of a nerve which is a branch of the mantle-nerve (Carriere, Patten, Rawitz). 



The eye which thus arises shows some similarity to the compound eye of 

 the Annelida as recently described by Andrews.* These eyes of the Lamelli- 

 branchia cannot well be compared with the compound eyes of the Arthropoda, 

 since the latter are far more complicated in structure. It is evident that 

 in neither case can there be any real homology. 



The Eyes of Pecten. The mantle-eyes of Pecten, the morphology 

 and physiology of which are still somewhat obscure, were investigated 

 from the ontogenetic point of view by Patten (No. 39), but his study 

 of them was not altogether satisfactory, so that we must content 

 ourselves with a short reference to them. 



The eyes of Pecten, unlike the two modifications of the edge of the mantle 

 just described in other Lamellibranchs, are highly developed organs (Fig. 

 28). The principal constituents of the eye of Pecten are as follows : there is a 

 cornea behind which lies a large lens ; behind the lens comes a retina com- 

 posed of a ganglionic layer, followed by a layer of rod-bearing cells, the most 

 remarkable feature of the retina being that the rods are directed away from 

 the light and towards the posterior wall of the eye. This latter is covered 

 behind by an integument of pigment-cells, in front of which lies the tapetum, 

 which has a metallic lustre. The innervation of the eye is double, and takes 

 place by means of a nerve (Fig. 28, 7), which sends out one branch to the 

 base of the eye, and thence direct to the optic cells, while the second branch 

 enters the eye laterally, becoming connected first with the ganglionic layer, 

 and through it coming into contact with the optic cells. For the further com- 

 plications found in this eye we must refer our readers to the special works of 

 Carriere, Butschli, Patten and Rawitz. 



Patten was able to establish ontogenetically that the eyes at the 

 edge of the mantle in Pecten arise as knob-like thickenings of the 

 ectoderm. As these thickenings rise up and become more and more 

 distinct from the surrounding ectoderm, an ectodermal cone grows 

 down from the surface towards the interior. While active increase 

 in number of the cells brings about the growth of the whole struc- 

 ture, the ectodermal mass directed inwards becomes marked off from 

 the outer epithelium, a process which is assisted by the growth of 



* Compound eyes of Annelids, Journal of Morphology, Vol. v., 1891. 



