in the Lamellibranchiata. 179 



Dr. Williams has made this mistake^ rriiay be pei?mittcd to say 



that he has not correctly stated the opinions there advocated. 

 The letters arose from the denial by Mr. Clark of the correctness 

 of a statement I made in a notice of Kellia rubra y that the ante- 

 rior tubular fold in that species performed the office of an ingress 

 siphon. The controversy arising out of it led to the expression 

 of our different views on the theoiy of branchial and siphonal 

 currents. I contended for the reception and discharge of these 

 currents by separate apertures (or in separate portions of the 

 cloak where two apertures do not exist), and that by the action 

 of cilia; but not by the ciHa lining the siphons alone^ as will be 

 evident from the following extracts. In the first letter my views 

 Are thus stated: — "The inhalent is always kept distinct from 

 the exhalent current, and admitted by a separate aperture from 

 that by which the latter is expelled. This seems to be neces- 

 sary, as the currents, being caused by the motion of the branchial 

 cilia, and not by the expansion and contraction of the walls of a 

 cavity, are continuous in one direction*/' In my second letter I 

 add a note to this effect : — '' The internal surface of these siphons 

 is usually (perhaps always) covered with vibratile cilia, more 

 minute than those of the branchiae, but acting in conjunction ivith 

 them in producing the currents. Mr. Cocks informs me that he 

 can see the cilia in the anterior tube oi Kellia suborbicularis, with 

 a lens of [ inch focusf." These statements surely cannot be 

 taken to imply that the currents are produced by the cilia lining 

 the tubes alone; and where does Dr. Williams find Mr. Hancock's 

 "alleged demonstration of cilia" on these organs ? The exist- 

 ence of cilia lining the internal walls of the siphons had been 

 J)i^viously noticed by Mr. Garner J and other authors; and not- 

 withstanding the decided opinion now given by Dr. Williams 

 against this view, I still believe that it is correct; but however 

 this may be, the well-earned scientific reputation of Mr. Hancock 

 cannot be brought in question by the result. 



With respect to the apertures by which the water is admitted, 

 T have stated, from the evidence of repeated observations (and in 

 this Mr. Hancock's observations agree with my own), tbat the 

 regular current for the supply of the branchiae passes in by the 

 branchial siphon, when it exists, and instances are adduced 

 where, when a strong current was passing in by that siphon, no 

 motion of the water was perceptible opposite the pedal opening. 

 We have nowhere asserted, however, as stated by Dr. Williams, 

 ^thxit all the water travels exclusively alona; the inhalent siphon, 



* Ann. Nat. Hist. 2nd Ser. vol. iii. p. 384. 



t 2n(l Ser. vol. iv. p. 51. j . ,^ , 



X Charlesworth's Mag. Nat. Hist. vol. iii. p. 298. 



12* 



