342 Professor Huxley [May 11, 



situated between the flat face of the adductor muscle, behind and 

 below, and the mass of the digestive viscera in front and above. It 

 consists of a large dark-coloured auricular division (aw.), partly 

 divided into two, which is situated below and in front and which com- 

 municates by two short tubular passages with the pear-shaped ven- 

 tricle («?.), the long axis of which is directed upwards and backwards. 

 Large arterial trunks are continued from the ventricle, one upwards 

 and backwards along the posterior moiety of the circular loop of the 

 intestine, one forward along the anterior moiety, one downwards to 

 the adductor. The successive contractions of the auricle and the ven- 

 tricle may be readily seen in the living oyster. The blood is colour- 

 less and contains numerous colourless corpuscles. It is conveyed by 

 the arteries to all parts of the body and thence proceeds to a large 

 venous canal, which lies in the middle line of the anterior face of the 

 trunk. From this it passes through the renal organs to the gills, and 

 is thence returned by a main vessel on each side to the auricular 

 division of the heart. The branchire consist of the four sickle-shaped 

 plates already mentioned, which extend, in pairs, from the palps in 

 front and above, to near the level of the vent behind (Fig. 2 (A) hr.). 

 Unlike a sickle, however, it is the convex edge of each which is sharp, 

 while the concave edge is broader. Each plate or lamella, as we have 

 seen, is V-shaped, consisting of two lamince which bound the intra- 

 lamellar cavity, and join below to form the edge of the lamella. It 

 can be shown that each gill plate answers to half the gill of those 

 Lamellibranchs in which the structure of the branchia retains its 

 primitive simplicity. Consequently the oyster has two gills and each 

 lamella is a hemi-hranchia made up of two lamince. Of the three 

 partitions which separate the supra-branchial passages, the right and 

 left represent the stems of the branchiae, while the middle one is 

 formed by the adherence of the edges of the inner laminae of the two 

 inner hemi-branchias to one another and to the anterior face of the 

 trunk. Even to the naked eye the surface of a hemi-branchia appears 

 marked with regular parallel transverse lines. And a low magnify- 

 ing power shows that these lines are the optical expression of a series 

 of parallel foldings of the lamina. The re-entering angles of the oppo- 

 site folds correspond and are united together for some distance, so that 

 the intra-lamellar chamber, or cavity of the hemi-branchia, is divided 

 into a series of parallel transverse tubular cavities, which are widely 

 open above, but which narrow and apparently become closed below. 

 The lamina itself consists of close-set parallel branchial filaments. 

 Each of these filaments has the shape of a lath, about y^V o *^ ^^ ^^ ^^^^ 

 thick, and five or six times as wide ; and they are set edge-wise with 

 their flat faces not more than ^^Jg-Qth of an inch apart. At intervals, 

 transverse bands unite these lath-shaped branchial filaments together. 

 The outwardly turned edges of the " laths " are closely beset with very 

 long vibratile hair-like processes, known as cilia; and, during life, 

 these work in such a fashion as to drive the water through the narrow 

 clefts between the branchial filaments into the cavities of the tubes, 



