Eycs of Molluscs and Arthropods. 647 



their exact number or shape. Jiulgiug- from the nuclei, tliere are at 

 least six browu pigment cells for cach ommatidium. Two prominent, 

 black pigmcnt cells coniplctely surround the neck of the caljx. In 

 lougitudinal sectious, or in isolated preparations, it is seeu that their 

 expanded and deeply pigmcnted inner extrcniity euds squarely, and 

 coutains a round nueleus : in the opposite direction they become thin 

 and narrow, and the pig-ment, which is diminished in quautity, tìnally 

 disappcars (fig. 1 18, pg. 3). The style of the retinophorae is completely 

 enclosed by seven retinulae, whose outer ends, in which the nuclei are 

 situated, are completely impreg-nated with nearly black pigment, while, 

 towards the inner ends, the axial faces alone are colored in this manner. 

 Three of these cells are longer and more deeply pigmented than the 

 others, so that a cross section of the ommateum, just below the calyx, 

 shows only the ends of these three cells arranged as in flg. 123; at the 

 very ends they are broad and thin, increasing in thickness, inwards. 

 All seven retinulae form a long double cone, whose g-reatestdiameter is in 

 the part where there is the most pigment. 



The Spaces between the inner ends of the ommatidia are occupied 

 by irregulär, slightly pigmented cells, containing oval, and distinctly 

 visible nuclei (tìg. 118, t/). 



Numerous, line, tracheal branches, tr, are found between the 

 ommatidia. 



The ommatidial layer of the eye rests upon an enormously thicken- 

 ed, conuective tissue, basal membrane, which forms a thick, sieve- 

 like layer permeated by canals, correspouding in number with the 

 ommatidia , and through each of which passes a bündle of pigmented 

 nerve tìbres. The outer surface of the basal membrane, upon which 

 rest the inner ends of the ommatidia, is well supplied with pigment, 

 which obscures its liner structure. The inner surface is thickened and 

 forms a sieve-like membrane, which stains deeply inhaematoxylin, and 

 might easily be mistaken for a true basal membrane (fig. 127, a. c). The 

 latter is composed of more compactly united, connective tissue tibres, 

 and is sharply delined on its outer surface, while on its inner, the tìbres 

 of which it is composed become continuous with those irregularly eu- 

 circling the numerous »nerve Channels«. Towards the periphery of 

 the eye, this massive membrane, which contains many nuclei, 

 beeomes thinner, and, finally, at the beginning of the ordinary h}-])0- 

 dermis disappears. The rouuded outer ends of the retinophorae, where 

 the f(Jur large, oval nuclei are situated, contain a very thin, watery 

 fluid; anything equivalent to a crystalline cone being absent. The walls 



