474 THE NERVE DISTRIBUTION IN THE EYE OF PECTEN IRRADIANS. 



few drops of absolute alcohol, and then poured into xylol and filtered, was used for 

 a contrast stain. The use of an alcoholic dye which weakens or destroys the methylen- 

 blue impregnation was thus avoided. 



Reliable information of the structural elements that compose the eye of 

 Pecten can best be gained from material hardened and stained with special nerve 

 methods supplemented by general cytological treatment. An examination of mate- 

 rial thus prepared and of macerated specimens reveals the nervous as well as the 

 supporting tissue. Although I centred my attention on the nervous tissues, I also 

 recorded the condition of the other structures that were present. 



II. GENERAL DESCRIPTION OF THE EYE. 



The eye may be considered as composed of three divisions: an anterior, dioptric 

 division consisting of cornea, lens, and iris (PI. XXXIV, Fig. 1, crn., Ins., ir.); a 

 median or retinal division enclosed in a retinal sac (sac. rtn.) ; and a posterior division 

 containing the vitreous humor, argentea, and red tapetum (hum, vit., arg., tap.). 

 Regarding the anterior division I can add but little to Patten's descriptions. Some 

 of the component parts were not exactly as Patten described them, but this no doubt 

 was due to the fact that the elements vary somewhat in different species. For instance, 

 in Pecten irradians the cells that form the cornea and iris do not possess the plicated 

 bases. The pigment layer, or red tapetum (Figs. 1, 2, tap.), that lines the posterior 

 part of the eye and the double-layered argentea which Patten described as independent 

 structures, I consider products of the same cells, and the vitreous network (Figs. 1, 2, 

 hum. vit.) is in my opinion nothing more than a fluid of the nature of a vitreous 

 humor. The red tapetum and double-layered argentea may be homologous to the 

 pigmented epithelium, the basal part containing the nuclei and stroma-like pigment 

 particles, while the argentea overlying the pigment corresponds to the clear refractive 

 distal part of the epithelial cells. It consists of layers that appear to be the result of 

 secretions of the pigmented red tapetum, formed of stroma-like particles devoid of 

 pigment and compressed into scales or platelets. The thinnest of these occur on the 

 distal side and like the proximal ones are regularly arranged in thin parallel layers. 

 They are traversed by the finest nerve-fibres, brought out by methylen-blue. While 

 some of the stains and hardening fluids act alike on the argentea, platelets, and stroma 

 scales of the pigment layer, either dissolving them in part and loosening their connec- 

 tions or leaving them unaffected by the stain, as hyaline refractile or iridescent scales, 

 others act only on the pigment-scales. The thinner peripheral layer may be torn in 

 sectioning from the proximal or both may be torn from the pigment-cells, in which 



