k 



SIPHONS OF BIVALVES. 63 



living Serpulm, the expansions of whose fans in cap- 

 tivity, and the use of the stoppers, — were highly 

 amusing. 



PHOLAS DACTYLUS. 



The respiration of many of the bivalve mollusca is 

 effected by means of a siphon, the two extremities of 

 which are situated close together, and are often 

 united so as at first to appear but one tube. A glance 

 at the very tip, however, even in this case, shews 

 that there are two openings, one of which is a little 

 smaller than the other, and commonly this subordi- 

 nate orifice diverges at a slight angle from the princi- 

 pal one. The latter is the entrance, the former the 

 exit for the water, a perpetual change of which is ab- 

 solutely indispensable to the life of the animal. The 

 interior of these tubes is said to be lined with innu- 

 merable delicate cilia; by the action of which the 

 surrounding water is drawn towards the entering ori- 

 fice, and conveyed in a strong current through the 

 tube over the surface of the gills. Then, having 

 been deprived of its oxygen, it is poured through the 

 other tube and expelled in a jet at its extremity, by a 

 similar machinery. 



This apparatus of double siphonal tubes is princi- 

 pally developed in those species which burrow, 

 whether in sand, mud, wood or stone. As the bur- 

 rowing bivalve usually, if not always, dwells in the 

 interior of the passage it has excavated, it is needful 

 that there should be a communication with the exter- 

 nal water, and hence a hole is always found extending 

 to the surface of the material bored. The entering 



