314 Mr. W. Clark on the Branchial Currents in the Bivalves. 



exhibit no appearance of a permeable structure; minute points 

 are scattered in the tracery of the parallelograms, some of them 

 being circled by a shallow grooved line ; but this is merely a de- 

 pression of the epithelium or its supporting membrane. I have 

 preserved the preparations. The P. Candida has not been exa- 

 mined, and the P. crispata does not inhabit the South Devon 

 coasts. 



Having mentioned accidental lesions and gill-laminar imper- 

 fections, I have to add, that in testing Messrs. Alder and Han- 

 cock's chief experiment, no alcoholic injections should be used, 

 as by their penetrating quality they may exude through these 

 supereminently delicate tissues; nor should mercury be employed, 

 as its weight in young subjects without great care often causes 

 ruptures, and from its density it does not pass near so freely as 

 aqueous fluids. Sea water coloured by indigo or archil, or pure, 

 is the proper injection, which must not be pushed beyond a fvdl 

 distension of the interlaminar tubes. The animal should be 

 prepared in as natural a state as possible, and not be killed by 

 any process producing sudden asphyxia, as immersion in hot 

 water or alcohol ; the first destroys tenacity in delicate tissues, 

 the second thickens and hardens them too much, and occasions 

 lesions and fissures by contraction. There must be no lesions 

 in the gill-laminse, except those that result from imperfections, 

 which prevail to more or less extent in every animal I have 

 examined — at least 500; any solution of continuity at the junc- 

 tion of the gills with the excessively delicate membranes of the 

 body will be fatal to success. 



If the experiment is thus conducted, no injection through the 

 anal siphon will flow into the branchial vault by the route of the 

 interlaminar canals ; the only moisture, if any, that can arrive 

 there, may be a slight exudation, a proportionate one to the 

 number of perforations and cracks in the membrane from laminar 

 malformation, and of these only those which pass through into 

 the interbranchial tubes. There may be in the 40,000 paral- 

 lelograms in each gill, about twenty flaws or imperfections, and 

 I reserve the possibility that all or most of these may arise from 

 the manipulation of such delicate tissues. 



After all these incidents, how am I to explain the great 

 discrepancy between the experiments of the northern naturalists, 

 illustrated by their " 10,000 pores," and mine, by the impossi- 

 bility of causing fluids to issue from the interbranchial tubes 

 by percolation through the membrane on which the network of 

 the blood-vessels is spread ? But 't is said, the sight is keener 

 in the North than with us southrons. The only solution I can 

 offer is a mere guess, that the animals operated on by these 

 gentlemen, after being killed, and alcoholized to harden the 



