Vol. XIV. January, igo8. No. 2 



BIOLOGICAL BULLETIN 



GILL DEVELOPMENT IN MYTILUS. 1 



EDWARD L. RICE. 



In the present paper no attempt is made to give an exhaustive 

 treatment of the subject ; the intention is rather to summarize 

 briefly the work that has been done upon the development of the 

 lamellibranch gill, and to describe somewhat fully certain phe- 

 nomena in the development of the gill of Mytilus which seem 

 thus far to have escaped notice. A few incidental observations 

 on gill development in other lamellibranch genera are added. 



Terminology. 

 The term ctenidium is used in the following pages to designate 

 the entire respiratory apparatus of one side of the body. The 

 halves of each ctenidium are called, in accordance with common 

 though questionable usage, the gills, outer and inner respectively 

 according to position adjacent to the mantle or to the body mass. 

 Each gill, again, is composed of two lamellae, called respectively 

 the direct (or descending) and reflexed (or ascending). The same 

 terms are employed in describing the limbs of the filaments com- 

 posing the lamellae. It will be noticed that the reflexed lamellae 

 (and filament limbs) of the outer and inner gills are turned in 

 opposite directions — that of the inner gill toward the body, that 

 of the outer gill toward the mantle. The Mytilus gill is of a 

 very simple filibranchiate type in which the interfilamentary con- 

 nections (between neighboring filament limbs of the same lamella) 



1 The studies upon which the present paper is based were carried on largely in the 

 Marine Biological Laboratory, Woods Hole, Mass., and the Harpswell Laboratory, 

 South Harpswell, Me. I am happy at this opportunity to express my appreciation of 

 the courtesies and assistance extended by the directors of these institutions, Professors 

 C. O. Whitman and J. S. Kingsley, and by their associates. 



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