1883.] on Oysters and the Oyster Question. 341 



the gill plates hang down, like so many elongated Gothic sickle- 

 shaped pendants, from the roof of the branchial cavity, which is 

 formed by their conjoined edges. On the posterior side, the cavity 

 inclosed between the pallial lobes is deep below, but rapidly becomes 

 shallower above, where the lobes are narrowed to mere bands. The 

 two bands pass into one another at the posterior end of the dorsal 

 integument — and form a rudimentary posterior hood, the velamen 

 (Fig. 1 (A), ve), which is very large in many other Lamellibranchs. 

 The intestine projects beneath the integument as it runs obliquely 

 downwards, over the posterior face of the adductor, to end in the 

 short but prominent tubular vent. The posterior interpallial space 

 into which this opens answers to the cloacal chamber of other Lamel- 

 libranchs (Fig. 1 and Fig. 2 (A), cL). It is continued forwards, 

 between the trunk and the dorsal faces of the gills, into a long 

 supra-branchial chamber (Fig. 2 (A), su.br.ch.), which extends forwards 

 and upwards, in front of the trunk, as far as the anterior and superior 

 ends of the gills. For their anterior third, the dorsal edges of all 

 the gills become attached to the front face of the trunk. The supra- 

 branchial chamber thus becomes subdivided into four passages, 

 which end blindly in front. The intra-lamellar cavities, which are 

 inclosed by each V-shaped branchial plate, communicate, either 

 indirectly through these passages, or directly, with the supra- 

 branchial chamber, and this widens behind into the cloacal chamber, 

 which opens freely on to the exterior upon the posterior side of the 

 body. But the supra-branchial chamber, its passages, and the intra- 

 lamellar cavities, would be completely shut off from the infra- 

 branchial chamber by the walls of the gill plates, were it not that 

 these walls are perforated, like a sieve, by multitudes of very narrow 

 parallel slits. 



The mouth of the oyster leads into a wide gullet, which passes 

 back for a short distance, and then dilates into a spacious stomach, 

 the lower and anterior end of which is continued into the long 

 conical first part of the intestine, which passes downwards in front of 

 the adductor, closely applied to its anterior contour. The next 

 portion of the intestine then bends sharply upon itself and turns 

 forwards parallel with the first part ; crosses this on the right side, 

 runs up along the back of the stomach, bends forwards and down- 

 wards, and turning back on the left side of the stomach (thus forming 

 a completely circular loop) passes at first backwards and then down- 

 wards to the vent, the place of which has been already described 

 (Fig. 2 (A) ). 



The stomach and the circular loop of the intestine are surrounded 

 by a dark brown or greenish organ, the short branched tubules of 

 which unite into larger tubes or ducts which open into the stomach. 

 This organ is known as the liver — though it by no means exactly 

 answers to the organ so called in the higher animals, but secretes the 

 fluid which is the chief agent in digestion. 



The heart (Fig. 2 (A) ) lies in a spacious pericardial cavity (jpc.) 



