ß 5Q William Patten 



gives rise to a pigmeutless area betweeu the style and the retinulae 

 (fig. 125). Whether that is the normal condition, or anartificial productof 

 the reagents used, I am uuable to say. The retiuidium, besides beiug 

 euclosed in the calyx , is also surrouuded by a somewhat thicker and 

 structureless membrane, the retinidial sheath, formed by the 

 united outer ends of the seven retinulae, which are coutinued outwards 

 to the surface of the ommatidial layer as thin cell walls. The abaxial 

 face of each retiuula is provided with a number of longitudinal 

 nerve fi bres connected with each other by numerous, minute, circular 

 ones (fig. 118). Around the retinulae are several bacilli , the number 

 of which for each ommatidium I was unable to determine with certainty, 

 but I have counted eight in several instances, and think that that is the 

 normal number. They are much larger than the nerve fibres and are 

 undoubtedly the inner, stalk-like ends of the outer pigment cells, with 

 which they should correspoud in number. They are round , colorless 

 and refractive rods, surrounded by nerve fibres; the inner ends, which 

 rest on the basal membrane, expand into several root-like fibres, a 

 proof that they are the modified inner ends of hypodermic cells. The 

 nerve fibres surrounding each bacillus, supply the outer pigmented ends 

 of the same. The bacilli may be followed in longitudinal sections as 

 far as the outer ends of the retinulae, but, occasionally, one is seen to 

 extend as far as the inner row of nuclei of the brown cells [fig. 118, 

 h. pcf] : they probably belong to those pigmented cells, the nuclei of 

 which may be seen between the outer ends of the calyces. 



Through each canal in the basal membrane, passes a bündle of 

 nerve fibres which are pigmented towards their outer ends. Whether 

 the same breaking up of the nervous bundles to supply four diflferent 

 ommatidia takes place here, as in Penaens, or not, I am unable to say, 

 since the material at my disposai was neither sufficiently abundant, 

 nor favorable for the Observation of such minute details. At all events, 

 the fibres , or groups of fibres, may easily be seen passing to the indi- 

 viduai cells which constitute the ommatidia; they may then be followed 

 along the whole length of the same , either in cross or longitudinal 

 sections, or in the dissected and isolated cells. 



Chapter III. General Remarks ou the Mollusca. 



The term ommateum was first introduced by Lankester to de- 

 signate all the soft parts of the Arthropod eye, with the exception of the 

 cuticular leus. Carrière adopted this term, and added that of omma- 



I 



