THE LIFE-HISTORY OF NUCULA DELPHINODONTA. 363 



Nucula. The habits of Yoldia are such as to render the 

 formation of strong currents of water absolutely necessary, 

 for otherwise the mantle chamber would become clogged 

 with fiBces and dirt. The gill of Yoldia might have been 

 perfected for pumping water from a Nucula-like gill. It 

 would, however, be equally easy to account for the reverse 

 modifications when we consider what the formation of strong 

 currents of water by an animal entirely covered by a soft, 

 slimy mud would mean. If we follow the generally accepted 

 theory of the gill, the former change would seem more likely 

 than the latter, though it is quite possible that nothing like 

 a direct change from one to the other has taken place. The 

 generally accepted tlieory of the gill has grown up as the 

 result of structural and embryological considerations, and 

 but scant attention has been given to probable modifications 

 for the special purposes of the animals. Until we have 

 much more detailed knowledge regarding the habits of most 

 of the Lamellibranchs that have plate gills, and of some of the 

 supposed near relatives of these Lamellibranchs, it seems to 

 me that we lack the necessary data to give the derivation of 

 the gill with anything like accuracy. There is much in the 

 structure and embryology of Nucula that points to a gene- 

 ralised type, and in this much it seems natural to look at the 

 gills as primitive; but the gills of Yoldia — its undoubted near 

 relative — are so remarkabl}^ well adapted for the performance 

 of a special function, that it hardly seems sate to regard them 

 as slightly modified gills until there are more careful observa- 

 tions on the habits of other forms. I recognise fully the 

 mass of evidence in favour of the primitive form of the plate- 

 like gill. My only plea is for caution. 



Hypobranchial Glands. 



The epithelium on the inside of the posterior end of each 

 lobe of the mantle, and on the outer side of a corresponding 

 portion of the suspensory membrane of each gill, is glandular, 

 and has been termed the hypobranchial gland. When these 

 glands are present in Lamellibranchs, their secretions seem to 



