PHOLAS. 195 



every five minutes a powerful and copious jet from both 

 tubes, sometimes simultaneously, at others at intervals, and 

 that from the branchial tube in the Pholades is almost 

 always accompanied by a strong ejection from the pedal gape, 

 and also in the Veneres from the pedal aperture, though from 

 the absence of a gape in their shells it is not so visible. The 

 periodic times of the in- and out-flux, of whichever character 

 it may be, as the animal becomes exhausted, are more and 

 more prolonged ; they are only in vigour for twelve hours. 



What is the object of these copious and regular receptions 

 and expulsions of water ? Will not every reasonable person 

 acknowledge that they can only serve for branchial purposes 

 the receptions, to administer water to the gill-laminse, and 

 the expulsions to remove it when effete by the contraction of 

 the adductors of the valves and siphonal retractors ? In cor- 

 roboration of the above, I particularly refer to the Rev. James 

 Bulwer's account of the Isocardia cor, published in the 

 'Zoological Journal/ vol. ii. p. 258. Messrs. Alder and 

 Hancock cannot controvert these facts, and therefore in rela- 

 tion to my theory say, "This is, however, a special case 

 having nothing to do with the regular branchial currents, as 

 has before been pointed out to Mr. Clark." 



My opponents may find their special case a general law, 

 and their system of regular branchial currents an illusion. 



They, having discovered that no ascertained communication 

 [this is a condition of my theory] existed between the 

 branchial and anal chambers, thus express the fact : " We 

 certainly find no opening between the foot and the gills, nor 

 between the gills and the mantle ;" and in consequence of 

 their favourite doctrine being in jeopardy, they "found it 

 necessary to make a careful examination of the anatomical 

 structure of these animals," and have informed us of the 

 discovery of a channel, by declaring the gill-laminae and their 

 interbranchial tubes permeable, on which to them a most 

 important fact, if true they emphatically observe, " Thus 

 in an instant the secret was explained ; the currents commu- 

 nicate through minute openings in the laminae of the gill- 

 plates." 



o2 



