1882.] American Work on Recent Mollusca in 1881. 879 



to travel for its living. That every divergence from a true cone 

 would be an advantage and would lead to hereditary retention or 

 repetition of the divergence, and that spirality (as we know) must 

 necessarily result from any deviation from the straight cone 

 whether due to a mere accidental fracture or any other cause. 

 Knowing this and knowing that in most active mollusks gravity 

 could not act in the same way and direction for five minutes at a 

 time, owing to their changes of position, it does not seem that 

 there is any need of it to account for the development of the spi- 

 ral in the shells of free gasteropod mollusks. 



But whatever view may be taken of single details, Professor 

 Hyatt's paper possesses, like most of his writings, the invaluable 

 quality of arousing discussion, exciting interest and of suggest- 

 ing new lines of thought; and of such essays we cannot have too 



Although first printed in the Quarterly Journal of Microscopi- 

 cal Science (London, 1881) and the result of studies by a native 

 of Japan, K. Mitsukuri's paper " On the structure and significance 

 of some aberrant forms of lamellibranchiate gills " (Studies from 

 the Biological Laboratory, Johns Hopkins University, ir, No. 2, 

 pp. 257-270, PI. xix, Mar., 1882) may be considered as in one 

 sense American work, since it was done at the laboratory of an 

 American university and under the instruction and direction of 

 Professor W. K. Brooks. The author here considers the struc- 

 ture of the gills of Nucula and Yoldia and their relation to the 

 gills of other acephalous mollusks. He arrives at the general 

 conclusion that the Lamellibranchiate gill was perhaps originally 

 a simple ridge on the side of the body, but to increase the surface 

 of contact with the water, folds may have arisen on two sides of 

 this ridge. If this be true, Nucula and Yoldia have advanced so 

 far as the gills are concerned, but very little beyond the primitive 

 condition. In course of time, however, as some forms of Aceph- 

 ala became less capable of extensive locomotion, these folds were 

 perhaps prolonged to form tentacular filaments, from which were 

 finally evolved complex gill structures like those of Mytilus, 

 Unio and Ostrea, which took on other functions than respiration, 

 such as assisting in the food supply by means of the currents 

 generated by their cilia. Between the simple gills of Nucula and 

 the complex ones of Unio, there are many intermediate stages 

 with modifications in different directions. 



