GASTROCH^NA. 269 



circular pipe, terminating at one end in a fringe and at the 

 other in a perforated disc, nearly resembling the rose of a 

 watering-pot. In the Clavagella one valve is loose in the 

 tube, and the other forms part of it; while in Fistulana^ as 

 well as in our British genus, both valves are loose within the 

 tube. 



M. Deshayes has entered into a very interesting inquiry 

 as to how the tube becomes enlarged, so as to correspond 

 with the increasing size of the animal ; and how the valves, 

 in the case of Clavagella and Asjjergilhmij become a part of 

 the wall-lining. That accurate naturalist endeavours to ex- 

 plain this, by supposing the animal to dissolve and recon- 

 struct successively the parts of the tube, to meet its altered 

 requirements. He believes the animal to secrete an acid 

 for the purpose of dissolving parts of the tube at pleasure, 

 and thus accounts for the disappearance of that part of it 

 which is now occupied by the fixed valves. With the 

 latter part of the supposition, our only genus of Tuhicolce 

 has nothing to do ; but as to the enlargement of the tube, 

 I may venture to remark, that the same principle appears to 

 be carried out with regard to the Tuhicolce as with regard to 

 the Teredo and the tube-making PJwlades. In a young 

 state, and during the period of growth and the labour of ex- 



