THE LTFE-HISTOEY OF NUCULA DELPHINODONTA. 315 



specimens in shallow dishes of sea water. When specimens 

 are placed on mud they bury themselves so promptly that 

 the movements of the foot cannot be carefully followed. 

 The movements are all such as would be of service in bur- 

 rowing in mud. Although specimens have been kept under 

 observation under different conditions for long periods of 

 time, I have never known one to execute movements that 

 could be interpreted as creeping. In 1853 Forbes and 

 Hanley, in describing Nucula nucleus (4), made the fol- 

 lowing statement: — "The foot is white^ and as if peduncu- 

 lated and deeply grooved, so as to expand into a broad leaf- 

 shaped disc with serrated margins ; by means of this organ 

 it can creep like a Gasteropod, and we have seen it walk up 

 the sides of a glass of sea water.^' This seems to be the 

 only observation of this kind on record, although many 

 students have worked on this and related forms. The 

 authors who have adopted the view that the foot functions 

 as a creeping organ in members of this group have, in nearly 

 every case, had only preserved material to work upon, and 

 perhaps have been influenced by finding so many characters 

 that seem to them to denote generalised structure. Some 

 Lamellibranchs are able to pull themselves over smooth 

 surfaces, but my observations lead me to believe that the 

 form and structure of foot found in this group is especially 

 poorly adapted for such a purpose (3). The expanded foot 

 of Nucula delphinodonta is relatively very large, and 

 the almost spherical shell is frequently turned from one side 

 to the other, but nothing comparable to creeping has been 

 observed. 



Although many Lamellibranchs carry their eggs and de- 

 veloping embryos, I think this is the first case reported 

 where a special external sac is formed for the purpose. This 

 sac (fig. 1) is composed of a mucus-like material, mixed with 

 foreign bodies, and is attached to the posterior ends of the 

 valves of the shell. Although the process of making the 

 sac has never been observed, it seems probable that the 

 mucus-like material is secreted by the hypobranchial glands. 



