206 PHOLADID. 
cannot be the agents of a communication from the branchial 
chamber to the anal siphon. 
It is necessary to state that occasional lesions, and now and 
then a perforation, are seen on the surface of the gill-lamine, 
the evident effect of a casual imperfection; with these excep- 
tions, entirety is the ruling aspect; all my fellow-observers 
concurred in this opinion; and two pieces of gill-lamma 
containing several interbranchial tubes were submitted to a 
distinguished metropolitan microscopist, who thus reported 
on them: “TI can find no pores in them, unless a piece of 
leather may be called porous.” Since this opinion a great 
number of the gill-membranes of the Pholas dactylus have 
been examined by transmitted light by one of Mr. Ross’s 
microscopes, with the $ and } of an inch object-glasses, a 
power more than sufficient to detect the presence of natural 
symmetrical apertures or pores through which effective per- 
meation could be obtained; indeed that power would be equal 
to show pores through which no water could pass freely, and 
scarcely by exudation. 
The gill-plates of the Pholas parva are more delicate than 
in the ‘ dactylus.’ No appearance of symmetrical apertures 
exists, but only an excessively minute wiry tracery, studded 
in the interstices with points, which, under a power of 300 
linear, only presented a surface little larger than the pomt of 
the finest needle, and had the aspect of prominent dots rather 
than pores. 
In the Pholas papyracea the gills are of the finest texture, 
but 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 depression of the epithelium or its supporting mem- 
brane. I have preserved the preparations. 
Having mentioned accidental lesions and gill-laminar im- 
perfections, I have to add, that in testing Messrs. Alder and 
Hancock’s chief experiment, no alcoholic injections should be 
used, as by their penetrating quality they may exude through 
these super-eminently delicate tissues ; nor should mercury be 
