of the Bivalve Mollusca. 243 



extended to their full length, an action commenced in the sur- 

 rounding water which was very discernible with a common lens ; 

 but for its more careful examination I put the animal under a 

 low power of the microscope, and could then distinctly see that 

 a current of water was passing in at the lower side of the open 

 mantle, partly by the cirrigerous portion (as observed by Cuvier 

 and others in the common mussel), but more especially at the part 

 of the mantle just in front of the cirrhi, and between them and the 

 foot. At the same time a very strong current was flowing off" by 

 the posterior siphon ; — so strong as to communicate a motion in 

 the same direction to the surrounding water and its contents. 

 These two currents continued while the mantle was expanded, 

 but on its withdrawal they ceased, and the animal became quies- 

 cent. During the whole of the time the valves remained sta- 

 tionary. 



My next experiment was upon Modiola nigra, and with exactly 

 the same results. The mantle of this species has the margin per- 

 fectly smooth, and is extended in the posterior part of the large 

 opening so as to simulate a second siphon. The current, how- 

 ever, did not go in at the prolonged extremity of this siphonal 

 fold, but at the anterior part of it. The egress-current of the 

 anal siphon was very distinct. 



A specimen of Mactra elliptica was some time in protruding 

 its siphons, which, as is well known, are long, and united to their 

 extremities. No distinct action of the water could be observed 

 until these were fully extended, and the hyaline valve exserted 

 from the anal siphon. A violent agitation then commenced in 

 the vicinity of the apertures, and, on looking carefully, I could 

 see a current containing floating particles, animalcules, &c. flow- 

 ing in at the branchial or inhalant siphon ; while an ex-current, 

 still more conspicuous, flowed simultaneously from the anal one, 

 sending the water to a considerable distance. At short intervals 

 during this operation a spasmodic contraction of the valves and 

 siphons sent off the water with a squirt ; probably at both aper- 

 tures, but this I could not distinctly make out. At such times 

 only was there any perceptible motion of the valves, which, while 

 the regular branchial currents were flowing, remained stationary 

 and were held a little apart. The water remained motionless 

 opposite the pedal aperture. The strong currents at the extre- 

 mities of the siphons induce me to attribute a more powerful 

 action to the cilia lining these orifices than I was at first inclined 

 to do, as they are generally much smaller and more difficult to 

 observe than those on the branchiae. 



The only other species I shall here notice is the Turtonia mi- 

 nuta. At first the water was observed to pass into the widely 

 open mantle of this little mollusk at all parts of the base of the 



