Eyes of MoUuscs and Arthropods. 685 



and contain a highly developed, and an aborted, nucleus, vve must ad- 

 mit tbat the possibility of reg*ardiug the phaosphere, foimd in Euscor- 

 pius Italiens^ by Lankester, as an aborted nucleus, is not so remote 

 as he would bave us believe. I bave omitted this factor in my deduc- 

 tion, for the sake of simplification : for, eveu if it were so, it woukl 

 not materially alter the eonclusions at whicb we bave arrived. 



It was not until I had definitely decided, by comparison, wbat must 

 bave beeu the eharacter of the primitive x\rthropod eye, tbat my atten- 

 tion was called to the eye of Perijxäus as described by Carrière. This 

 type of eye resembles so perfectly the presumed ancestral oue tbat 

 either migbt be regarded as the primitive form. 



This eonception, theu, is of double value, tirstly, because, according 

 to it, all the various forms of Arthropod eyes (witb the exceptions al- 

 ready mentioned; may be regarded as modifications of a single, primi- 

 tive type, exactly like tbat found in the most primitive Arthropod ; se- 

 condly, because the so-ealled Molluscau eye of Peripatus is no longer 

 remarkable on account of its dissimilarity witb other Arthropod eyes, 

 but, in fact, is completely identical witb wbat we bave good reason for 

 supposing is the primitive Arthropod eye. 



In the Crustacea, the number of cephalic eyes is never so great 

 as tbat often found in the Insecta, and in by far the majority of cases is 

 limited to two. There is seldom more than a single, median, larvai eye, 

 probably representing a fusion of two paired ones, whicb are very 

 rarely retained in the adult. 



Euphausia is remarkable on account of its numerous, segmentally 

 arranged eyes, or luminous orgaus, asSARs' considersthem. Through the 

 kindness of Dr. Paul Mayer, I bave had the pleasure of examining some 

 very carefuUy prepared sections of these organs. I hoped to give a more 

 complete description of them, but tbat must be reserved for some future 

 time. 



Sars States tbat there is no reason to consider them as visual 

 Organs, since their structure is not like tbat of eyes in general. I 

 cannot agree witb bim to this respect. I do not think Sars examined 

 the eyes by means of sections , witbout whicb it would be extremely 

 hazardous to form an opinion. The sections I had the pleasure of 

 studying shoAved a complete agreement in essential characters witb the 

 visual Organs of other animals. The lens is separated from the exte- 

 rior by a double layer of cells, and is suspended in a well-developed 



1 Keport on the Schizopoda collected by H. M. S. Challenger diiring the 

 years 1873—1876. in : Challenger Reports Vol. 13, Part 37, p. 70—72, PI. 12. 



