BRANCH LE OF BIVALVES. 275 



The branchiae of the bivalved mollusca are always placed 

 between the body and its cloak, the folds of which being, 

 in many of them, altogether separate in front, admit the 

 circumfluent water very freely ; but when these folds are 

 soldered together at the edges, as they often are, the water 

 is imbibed through a branchial siphon formed by an elonga- 

 tion of the cloak, and extruded at the posterior end of the 

 shell ; and the effete fluid is expelled again through another 

 or anal siphon, — the common excrementitious tube, which, 

 in general, is altogether like the other, and occupies the 

 same position.* All bivalves which burrow in the sand or 

 mud are furnished with these siphonal tubes. They are 

 either separate or they are bound together in a common 

 sheath ; and their capability of elongation is always in 

 exact accordance with the depth of the furrow of the 

 mollusk : — if this merely covers itself with the soil the 

 tubes are short and little extensible, — if it burrows deep 

 they are made protrusile to the requisite extent. Their use 

 is to keep up a communication between the animal and the 

 superincumbent water, and the interruption of that com- 

 munication would have the same deadly effect as the depri- 

 vation of air from any terrestrial animal. To prevent this 

 death, and to allow a free flow of water unto them, there 

 are always holes in the sand corresponding to the apertures 

 of the tubes ; and you must further notice that these aper- 

 tures, more especially the branchial one, are encircled with 

 a series of tentacular filaments that prevent the ingress of 

 all noxious matter, and strain the water of respiration if 



* This is the view generally received by malacologists, but Mr. W. 

 Clark, an excellent observer, affirms that the circumfluent water enters by both 

 siphons, and is also expelled through both of them. " I have little doubt 

 that the water, required for buccal and branchial uses, in the mollusca with 

 closed mantles, is received through both the posterior apertures, anal and 

 branchial as they are called ; and probably at their bases there is an internal 

 communication, thus allowing the water from both to pass into the great 

 cavity of the branchiEe, to bathe them, and for sustentation of the animal ; 

 and after these functions are fulfilled, it is in like manner expelled from 

 both orifices, and often simultaneously, as may be seen in any of the Pholades, 

 Lutrarite, or Myae." Ann. and Mug. N. Hist. scr. 2, iii. 453. — These 

 views of Mr. Clark are controverted by Mr. Alder and Mr. Cocks, who 

 maintain the accuracy of the ordinary statement. Lib. cit. iv. 50 and 55. 

 Mr. Clark's observations coincide with those of Spallanzani, made on 

 Ascidise ; and in reiterating them, Mr. Clark says ; — " The assumed regu- 

 larity of the admission and discharge of the branchial currents is a sad 

 mistake ; nothing can be more irregular, capricious, and uncertain ; they 

 depend entirely on the volition, habits, and wants of the animal, and are 

 often suspended for weeks in Kellia rubra, and twice in every twenty- 

 four hours in the mussels and numerous Gasteropoda inhabiting the 



higher levels of the littoral zone." — iv. 143. 



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