152 S. WATASE. 



Finally, we have to consider the nature of the compound eye 

 as a whole as presented in various types of Arthropods. 



That a certain structure in the body of an animal may 

 repeat itself and give rise to a secondary aggregate, or to 

 a compound organ, is a well-known fact; the repetition of 

 similarly constructed uriniferous tubules forms the essential 

 part of a Vertebrate kidney, or the similar repetition of gill- 

 filaments forms the respiratory organ of a Lamellibranch. 

 Sundry other examples of this nature might be given, but the 

 above two will suffice. Tracing, as I have attempted to do, 

 the most complicated omraatidium into a simple, open, ecto- 

 dermic pit, there is to my mind no difficulty in believing that 

 the compound eye of the Arthropod is one of the most astonish- 

 ing examples of the formation of an organ by the vegetative 

 repetition of the similar structure. Thus, according to 

 Lubbock, there are about 4000 facets in the compound eye of 

 the house-fly (Muse a), each facet corresponding to a single 

 tubular invagination of the skin, the ommatidium. There are 

 4-000 independent invaginations in the area in the head of the 

 fly occupied by the compound eye; in the gad-fly (CEstrus), 

 7000; in the goat-moth (Cossus), 11,000; in the death's- 

 head moth (Sphinx atropos), 12,000; in a butterfly 

 (Papillio), 17,000; in a dragon-fly (^schna), 20,000; in a 

 small beetle (Mordella), as many as 25,000. On the other 

 hand, the number of ommatidia seems to have reached its 

 minimum in certain Copepods, as in Corycccus, where the 

 whole visual organ seems to be represented by a single colossal 

 ommatidium. 



Certain forms of Collembola^ seem to have a very small 

 number of ommatidia; thus in Templetonia only one 

 ommatidium exists on each side of the head; Orchesella 

 has six on each side of the head; Tomocerus, Ipsoma, 



1 Lubbock, • Monograph of tlie CoUembola aud Tbysauura/ the Ray 

 Society, 1S73, p. 57, pis. Iv aud Ivi. Lubbock uses the term " ocellus " to 

 desiguate a single element of the eye, which I here called au ommatidium. 

 If the structure of this " ocellus " differs from the ommatidium of other 

 Arthropods, it has, of course, nothing to do with the discussion at issue. 



