MUSEUM OF COMrAllATlVE ZOOLOGY. 433 



ADDENDA AND CORRIGENDA TO PART L, 1886. 



The Structure of the Gill in Amusium Dalli (T. p. 209), Dimya argen- 



tea (I. p. 22S), and Area ectocomata (I. p. 213). — The structure of the 

 gill in Liuuellibraiichs is of considerable interest, and has been investigated by 

 Peck, Lacaze-Duthiers, Bonnet, Mitsukuri, and others. The subject is very- 

 far from exhausted, however, and progress in this direction has been checked, 

 not so much because a majority of the known forms have been investigated, 

 which will not be true for a long time to come, as because later students have 

 assumed that the few which they have examined adequately represent the 

 many which they have not seen. Another difficulty is, that so far no sys- 

 tem of nomenclature has been adopted, or even suggested, by which one could 

 designate a particular form of ctenidium when recognized. Dr. Paul Pelseneer 

 therefore deserves credit for attempting a diagrammatic classification in his 

 Report on the Anatomy of Deep Sea Mollusks in the Challenger series. 



Unfortunately, when he leaves the regions of research for those of hypothe- 

 sis, it is at once evident to any one having a wide knowledge of these organs 

 in the Mollusca that his basis is inadequate, especially if the theories be tested 

 by application to mollusks in general. Dr. Pelseneer has recognized neither 

 the multiplicity of form which is exhibited by molluscan breathing organs, 

 nor the inadequacy of our present knowledge as a foundation for such wide 

 generalizations. Consequently his discussion is chiefly valuable as calling 

 attention to the subject, and presenting a preliminary basis for future com- 

 parisons. 



In order to correct certain observations of my own, if they stood in need of 

 it, and to confirm them if sundry doubts of Dr. Pelseneer's proved ill founded, 

 I reviewed the material discussed in Part I. of this Keport. 



A section through the ctenidium, at or near the point where it becomes 

 free from attachment to the mantle, gives conclusive evidence of the gill struc- 

 ture at a glance. Putting this in practice, I examined the original specimens 

 referred to at the head of this page, and a variety of other species for purposes 

 of comparison. 



Beginning w^th the simplest form, Dimija, I found the original description 

 given by me to be in need of no corrections. The base of the gill consists of 

 a rather large tube, constituting the branchial vein or blood-sinus, with a 

 single series of filaments on one side of it, unconnected except at their base 

 of insertion. Prof. Huxley states that the simplest form of gill consists of "a 

 stem fringed by a double series of filaments" (Inv., p. 408, Am. edition). In 

 VOL. XVIII. 28 



