THE GILL- SAC. 301 



commonly both at the same instant. They are, how- 

 ever, soon opened again ; and we may discern, espe- 

 cially if the specimen is in a glass vessel, and we watch 

 it by the aid of a lens, with the light of a window at its 

 back, that a current of the surrounding water flows 

 from all sides to the taller orifice, and pours down its 

 tube ; while occasionally we see the ejection of a stream 

 from the orifice of the shorter tube. Thus we have here 

 a receiving and a discharging tube, the exact repre- 

 sentatives of the two siphons in such bivalves as 

 Pliolas, Venus, etc. The former leads down into a capa- 

 cious sac in the interior, the walls of which constitute 

 the breathing apparatus. The inner surface is marked 

 by regular parallel ridges which run in a horizontal 

 direction ; and these are again connected by vertical 

 ridges at right angles, very numerous, enclosing a vast 

 number of oval compartments. The sides of these are 

 richly ciliated ; and if the whole apparatus be carefully 

 dissected out, and laid upon the stage of the microscope, 

 the course of the ciliary currents may be distinctly 

 seen, continuing with unabated vigour and with unfal- 

 tering precision for a long time after the severance of 

 the organ from the body of the animal. But all this 

 is seen to most advantage, if we select one of the smaller 

 species, which are brilliantly transparent, such as one 

 which grows in groups of elegant tall vases, about an 



