^ THE SIPHON OF THE PHOLAS. 91 



tried him in vain. He lolls his great length at the 

 bottom of the pan, and declines the Imnp of wood 

 placed before him. In fact, he does notliing but suck 

 in the water at one tube of his siphon, and squirt it 

 out of the other. Do observe that siphon or double 

 tube, like a double-barrelled gun, the lining membrane 

 of which is covered with vibratile cilia. The incessant 

 action of these cilia draws the water in at the orifice of 

 the upper and larger tube, along which it passes and 

 reaches the gills, where the blood is aerated ; and then 

 the water makes its exit from the under tube, in a 

 steady current, visible to the naked eye. How this is 

 performed is to me a mystery, for my dissections 

 wholly failed in tracing any direct communication 

 between the two tubes ; but that there must be some 

 indirect communication is certain, since the evidence 

 of the two currents, one of entrance, and one of exit, 

 is unequivocal.* Look at the orifice of the uj^per 

 tube, what a beautiful arborescent fringe encircles it ! 



* This disputed question has beeu finally settled by the investiga- 

 tions of Messrs Alder and Hancock, who find the communication takes 

 place through minute apertures between the meshes of the gills them- 

 selves. "Each of the gill-plates consists of two laminae united at the 

 ventral margin, and likewise attached to each other in transverse lines 

 running across the gills throughout their whole extent, and forming, 

 in the interspaces, a series of parallel tubes which open into the dorsal 

 chamber, and are thus in communication with the excurrent siphon. 

 The minute reticiolated blood-vessels of the branchial laminae forming 

 the walls of these tubes, are found, when examined by a high power 

 of the microscope, to be open between the meshes, which are minutely 

 ciliated, allowing the passage of the water into the tubes, and from 

 thence into the anal chamber." — Report of British Association, 1851 ; 

 Sections, p. 75. 



