40 Dr. T. Williams on the Mechanism of Aquatic 



plete demonstration. Mr. Clark*, however, embraces still the 

 doctrine which contends for the non-communicating independ- 

 ence of the siphous and of the cavities of which they are the ex- 

 ternal continuations. In this respect, his conclusions are directly 

 opposed to the results of the author's observations. That the cavi- 

 ties recently so clearly defined by Mr. Hancock are by structure 

 and office distinct, will be afterwards irrefragably proved. This 

 division of the pallial enclosure into two leading sub-areas con- 

 stitutes a fundamental feature in the oeconomy of the Tunicate 

 and Lamellibranchiate mollusks. It suggests a natural process 

 of thought by which the siphonic actions are interpreted infal- 

 libly. It will be subsequently proved, that that siphon which is 

 said by Messrs. Alder and Hancock to give ingress to the water, 

 is really no more branchial than that by which the fluid makes its 

 egress. Both bear to the branchiae the same anatomical relation. 

 It were as correct to designate the opercular orifice in the fish as 

 the "anal," and the mouth as the " branchial/' as to apply such 

 terms to the siphons of the Tunicata and Acephala. Such de- 

 signations misinform. They express either what is not true, or 

 what is only partially true. The " branchial " siphon is as much 

 oral or prehensile as branchial. The " exhalent " as much anal 

 as expiratory. 



It is quite established that two distinct offices devolve upon 

 each siphon. The one is designed to take in water for the pur- 

 pose of breathing, and alimentary particles for the purposes of 

 food ; the other emits at once the products of the respiratory 

 and digestive processes. One name as applied to either will not 

 express the double function. Let the name therefore be drawn 



the pallial from the anal chamher. 2ndiy. That by this act of sieving the 

 food, the aliment is separated from the water and impelled by ciliary action 

 towards the free margins of the gills and along the groove formed expressly 

 for this purpose on this margin, and finally borne in the direction of the 

 mouth. And 3rdly. The distinctness of the inhalent from the exhalent 

 current; while Dr. Sharpey speaks plainly upon the point that the ex- 

 current is set in motion exclusively by the branchial cilia. It is extraordi- 

 nary that, in asserting claims to originality upon these very points, in 

 papers published ten years afterwards, so careful and honest a student as 

 Mr. Hancock should have permitted this accessible and celebrated article 

 of Dr. Sharpey to elude his literary search ! I rejoice rather than lament 

 over Mr. Hancock's " sin of omission." Confirmation, enriched bv nume- 

 rous valuable original details, proceeding from so truthful an observer, 

 must prove of immense service to the cause of science ; but, palmam qui 

 meruit. To widen the bounds of knowledge is the highest gratification 

 which belongs to the true man of science. This is his most valued title of 

 nobility. To withhold from the labourer his just reward, is to perpetrate a 

 criminal offence against science. 



* On the Pholadida?.— Ann. and Mag. Nat. Hist. Nov. 1850. 



