624 William Patten 



pods, it is simply necessary to kuow what are the rods, and by what 

 cells they are prodiiced, whether by tbe pigmeuted, or eolorless ones, or 

 by both. The presence or absence of pigment is a factor altogether toc 

 iincertain to be, alone, of any value ; for, as I have shown in Haliotis, the 

 double cells, or those which are nsually eolorless, may sometimes con- 

 tain a small quantity of pigment, while in Pecten both elements are 

 perfeetly devoid of pigment. Still auother factor must not be ne- 

 glected, and that is that in the Cephalopods the pigment between the 

 rods and aronnd their axial nerve fibre is something entirely nnique. 

 The presence of pigment aronnd the axial nerve fibre is of great theo- 

 retical importance, for, if pigment may be produced by a nerve fibre 

 within the rod , there is no reason why the externa! fibres of the rod, 

 arising from the limiting or ganglionic cells, may not also produce pig- 

 ment. Moreover, the manner in which the axial nerve is protected from 

 the light proves that it cannot be the percipient dement, while, on the 

 other band, the access of light to the walls of the rods containiug the 

 retinidial cross fibrillae shows that there , as in other Mollusca, is the 

 true seat of the light-sensitive elements. 



Grenacher (39) inconsistently speaks of the half of each rod as a 

 rhabdomere, which, according to bis definition, should be the product of 

 a single cell. The so-called rhabdoms found by Grenacher in the Ce- 

 phalopods bave absolutely no morphological signification. They are 

 simply due to a deceptive, but economic arrangement, so that the broad 

 sides of four different rods are adj acent to each other. The integrity 

 of the individuai rods, which are similar to those of Pecten^ is in 

 no wise aifected by this arrangement. It is impossible to find any 

 points of resemblance between such a rhabdom and that of any 

 other animai. He has hastened to apply bis name rhabdom to 

 these accidental groups of rods, regardless of the consequences. When 

 a person applies terms, necessarily restricted in meaning, to widely 

 difierent objects, oue is led to believe that there must be some resem- 

 blance between the objects in question, and possibly some morpho- 

 logical relationship. But in the present case (p. 251), he is neither 

 able to find any resemblance between bis rhabdoms in Cephalopods 

 and Arthropods, nor is he willing to admit that any morphological 

 value could be attached to such a resemblance, provided it existed. 



Without further discussion of the views either of Grenacher or of 

 BüTSCHLi, I may be permitted to draw comparisons between the retina 

 of Cephalopods and that of other Mollusca, based upon my own re- 

 searches. The whole question hinges upon the nature of the so-called 



