204 PHOLADID A. 
Since May 1853 I have often performed “the simple ex- 
periment” detailed by Messrs. Alder and Hancock in the third 
paragraph of their paper, p. 374; it is by far the most im- 
portant of the series, as the problem of communication, with 
them, between the anal and branchial siphons, depends on it. 
By the imjections of more than 200 Pholades with mercury 
and coloured fluids, the invariable result has been my mability, 
as in the first experiments in 1850, to pass the fluids through 
the anal chamber further than to fill all the mterbranchial 
tubes; but I always found the gill-laminz, which form their 
walls, impervious, instead of allowing liquid to issue “from 
10,000 pores.” It is necessary to state that the numerous 
interlaminar canals that compose the divisions of the gill- 
plates are nearly parallel, and hang vertically from the dorsal 
line, ranging at equidistances throughout a great part of the 
extent of each branchial plate, and by sutural lines of junction 
cut off the communication between each tube. 
I will now enter a little more into detail on some points in 
connection with the branchial laminz, by describing the ap- 
pearance of the areas of the parallelograms under repeated 
examinations by transmitted light, and also as opake objects, 
rendered so by the injection of mercury. 
In a full-grown Pholas dactylus, the surfaces of each gill- 
lamina together comprise an extent of about a square inch, 
every one-tenth of which contains 400 oblong subquadrangular 
spaces, or 40,000 in each plate, forming a total in the four 
gills of 160,000; this admeasurement and enumeration may 
not be very far from the truth. In each parallelogram, besides 
a general suboval depression, there are within it from five to 
twenty or more shallow excavations of various size and shape, 
but there is no ruling symmetrical fissure as delineated in 
Messrs. Alder and Hancock’s fig. 3. Each area shows a plain, 
a pitted, and a mammillated or traceried surface, detected by 
the action of the microscopic foci. We will start from the 
plain surface, in which there is certainly no perforation: the 
fine adjustment of the imstrument measures the depth of the 
depressions, and by another movement shows the character of 
the minute points, thus proving that no fissure or aperture 
