296 ANDREWS. [Vol. V. 



1 



The occurrence of stiff, hair-like processes upon these organs 

 as observed by Semper (6), by Ehlers (8) in Myxicola and 

 Filigrana, and their possible existence in Potamilla and others, 

 raises a suspicion that some other sense organ in addition to or 

 in place of an eye may be here concerned. 



The great number and position of these organs suggest 

 doubts as to their usefulness as eyes : the same that have been 

 made to the like organs of Area. 



Finally, a much more important fact is the dubiousness of 

 our present evidence. In Potamilla, as in so many other cases 

 observed by others, the animals are exceedingly sensitive to 

 sudden passage of very slight shadows over their branchiae. 

 Almost always, but with striking exceptions, they instantly with- 

 draw into their tubes. The natural assumption that the peculiar 

 branchial organs are here concerned is no proof. In fact, 

 Hydroides dianthns is found to respond fully as well, though, 

 as we have seen, it has no such branchial organs nor any other 

 special sense organs as far as known. 



Here the seat of sensation is also in the branchiae ; when 

 these are cut off more and more, the animal still reacts till 

 nothing but the bases of the branchial stems remain. 



As the sensitiveness attributed to the branchial organs may 

 be thus present in other undetermined parts of the branchiae, 

 we must fall back for the present upon the structural evidence 

 and upon comparison with other groups to support our belief 

 in the existence of functional, compound Annelid eyes. 1 



Baltimore, Feb. 12, 1891. 



REFERENCES. 



1. Sir J. G. Dalyell. The Powers of the Creator, etc. London, 1851-58. 



2. A. KoixiKER. Ueber Kopfkiemen und Augen auf den Kiemen. Zeit. 



f. wiss. Z00L, IX, pp. 536-541. 1856. 



3. Ed. Claparede. Anat. Ent. wirbl. Thiere Normandie. 1863. 



4. Id. — Annelides Che'topodes du Golfe de Naples. 1868. 



5. J. Chatin. Recherches pour servir a l'histoire du batonnet optique. 



Am. Set. Nat. Zool., Ser. 6, I-VII. 1878. 



1 Experiment with Potamilla with reference to the possible phosphorescent nature 

 of the branchial eyes gave a negative result. 



