206 PHOLADID^E. 



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-laminse, 

 the evident effect of a casual imperfection ; Avith these excep- 

 tions, entirety is the ruling aspect; all my fellow-observers 

 concurred in this opinion; and two pieces of gill-lamina 

 containing several interbranchial tubes were submitted to a 

 distinguished metropolitan microscopist, who thus reported 

 on them : " I 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 traceiy, studded 

 in the interstices with points, which, under a power of 300 

 linear, only presented a surface little larger than the point 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 structiu'e ; minute 

 points are scattered in the traceiy 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 



