204 PHOLADID.E. 



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 injections of more than 200 Pholades with mercury 

 and coloured fluids, the invariable result has been my inability, 

 as in the first experiments in 1850, to pass the fluids through 

 the anal chamber further than to fill all the interbranchial 

 tubes ; but I always found the gill-laminse, 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 laminae, 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 instrument measures the depth of the 

 depressions, and by another movement shows the character of 

 the minute points, thus proving that no fissure or aperture 



