38 Dr. T. Williams on the Mechanism of Aquatic 



cate the first as the orifice of ingress, and the last as that of egress. 

 This, however, is not the acceptation in which they are used by 

 authors of great celebrity. Mr. Rupert Jones* observes : " The 

 position of the animal is such, that of the two orifices the 

 branchial is always the highest ; the entrance into the branchial 

 sac being generally placed at or near the superior extremity of 

 the body, and the oesophageal opening at the base of the branchial 

 sac having an upward direction." This is directly opposed to 

 the definition of M. G. P. Deshayesf, who says — "Whether 

 connected or not, the superior siphon is always characterized as 

 the anal, the inferior as the branchial siphon." Of course the 

 comparative " superior " must mean that which is nearest to the 

 hinge or dorsum ; " inferior," that which is next to the venter, the 

 antipodal point to the hinge. The branchial siphon of Mr. U. 

 Jones is therefore correspondent with the anal of M. Deshayes. 



The expression of Cuvier — " deux ouvertures separees, Pune 

 pour la respiration, l'autre pour les excremens," &c. — suggests 

 the idea that one tube, the branchial, is devoted exclusively to 

 respiration ; that is, that through the same tube the inspiratory 

 and expiratory currents concerned in breathing take place. 



Dr. George Johnston observes : " The water is imbibed through 

 a branchial siphon. The effete fluid is expelled again through 

 another or anal siphon J." The branchial siphon of other authors 

 is the longest or superior, and is distinguished as that which 

 emits the refuse water which has traversed the branchiae. The 

 branchial siphon in the sense in which it is used by Mr. Garner § 

 is synonymous with the inhalent tube, and the anal with the ex- 

 halent. In this acceptation the terms are also used by Forbes 

 and Hanley||, by Alder and Hancock^, and by Mr. Clark 

 in his excellent controversial papers against Mr. Hancock in the 

 ' Annals/ Dr. J. E. Gray attaches to these words a similar 

 meaning, calling the inhalent ' the lower ' siphon, and the exha- 

 lent ' the upper'**. 



The " branchial " siphon of the most esteemed authors then 

 is that tubular extension (PI. I. fig. 7, a, a') of the mantle 

 by which the surrounding element is admitted into the " bran- 



* See article Tunicata. — Cyclop. Anat. Phys. 



t See the article Conchifera. — Cyclop. Anat. Phys. 



J See his recent excellent work, entitled ' Introduction to Conchology/ 

 p. 275. Van Voorst, 1850. 



§ Transactions of Zoological Society, vol. ii. p. 91. 



|| British Mollusca, vol. i. 



*jf See their valuable papers on the Branchial Currents in Pholas and 

 My a, Annals and Magazine of Natural History, Oct. and Nov. 1851. 



** See his original and instructive papers in recent Numbers of the 

 ' Annals,' on " A Revision of the Arrangement of the Families of Bivalve 

 Shells," &c. 



