THE LIFE-HISTORY OF NUCULA DELPHINODONTA. 365 



oflands, and I find that it does not form brood-sacs. Such a 

 case as this, where it is known that brood-sacs are not 

 formed, seems to indicate either that the glands have some 

 function to perform other than providing* the material for 

 the formation of brood- sacs, and that Nucula delphino- 

 donta has adapted them to this purpose; that they are 

 retained from forms that originally formed brood-sacs, in 

 which case we must suppose that the an'cestors of all forms 

 that possess hypobranchial glands formed brood-sacs; or 

 that in forms where brood-sacs are not formed the glands 

 are, when present, mere vestiges, and are not now functioiuil. 

 The latter explanation seems unlikely, as the glands of 

 Nucula proxima are better developed than vestiges are 

 likely to be. If the second explanation is accepted, we must 

 regard the rearing of embryos in brood-sacs as more primi- 

 tive, for this group at least, than throwing the eggs in the 

 water where the embryos have to take care of themselves. 

 From the standpoint of specialisation this seems to be very- 

 unlikely, find the fact that the embryos of Nucula delphi- 

 nodonta possess tests that seem to serve no purpose, while 

 similar tests function as organs for locomotion in other forms, 

 points clearly to a condition when all of these embryos de- 

 pended on their own activities for protection. It seems 

 most likely that Nucula delphinodonta has made use of 

 already existing glands to furnish the secretions for the 

 formation of its brood-sacs, and that they may have other 

 functions to perform. 



Pericardium. 



A short time before the shell-gland begins to fold at the 

 sides to form the lobes of the mantle, a space appears 

 between the stomodaeum and the gut, and a little later a 

 space begins to form between the shell-gland and the gut 

 (fig. 24). These two spaces are separated by the gut, but in 

 preserved material they are frequently connected around the 

 sides of the gut. While these connections may be due to 



