COMPOUND EYES OF ANNELIDS. 



E. A. ANDREWS. 

 I. POTAMILLA. 



Thinking the pecuHar pigmental organs upon the branchiae 

 of certain tubicolous, sedentary Polychaetae would repay investi- 

 gation, as being apparently compound eyes entirely different 

 from anything in the higher Annelids, and as possibly giving 

 aid in the interpretation of Arthropod eyes, I availed myself of 

 the opportunities offered at the United States Fish Commission 

 Laboratory in Wood's Holl, Massachusetts, to study several spec- 

 imens of Potamilla obtained there in the summer of 1889, and 

 again in 1890. 



The species examined is undoubtedly Potamilla reniformis, 

 of Malmgren, though the number of eyes may be as many as 

 seven or eight upon each branchia, while Malmgren figures only 

 three : small individuals have, however, as few as three, or even 

 one or two ; and others a variable number, even different bran- 

 chiae of the same individual presenting considerable variations 

 in this respect. Though this New England form was at first 

 referred to a new species, P. oculifera, by Professor Verrill, its 

 identity with the European is recognized by him in more recent 

 publications. 



This Annelid lives in a leathery tube projecting somewhat 

 from holes in gasteropod and bivalve shells, etc. From the end 

 of this tube the cephalic branchial plumes are expanded as a 

 circular series of radiating stems, each bearing two rows of bran- 

 chial filaments, all directed anteriorly in the fully expanded con- 

 dition, when the whole apparatus stands about at right angles 

 to the long axis of the body. In this state the eyes are upon 

 the posterior or outer sides of the main stems of the branchiae, 

 a row of three to eight upon each of the twenty stems. They 

 are not arranged with much symmetry, but yet tend to lie in 

 concentric rows upon all the branchiae collectively, since the 

 proximal ones occur at about the same distance out upon all. 



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