170 ANATOMY OF THE LAMELLIBRANCHIATA. 



low this orifice, and another above, to regulate the currents 

 in the proper direction. The orifice is lengthened into a tube 

 in the Lithodomus ; and in the Pinna the anal valve has ta- 

 ken a lengthened ligulate form. The Uniones only differ from 

 this in having the branchiae united, and the water appears to 

 make its exit by insinuating itself between them and the foot, 

 and so through the orifice. In the Cardium we see two dis- 

 tinct openings behind ; the water enters by the lower one, 

 distends the mantle, and this orifice being then closed by the 

 valve, it gets between the foot and the branchice, and is dis- 

 charged through the superior orifice. In this animal the pos- 

 terior extremity of the branchiae is united to the septum be- 

 tween the orifices, so that the two respiratory cavities only 

 communicate by the side of the foot. In the Cyclas both the 

 short tubes open internally below the branchiae, and there is 

 no passage required between the latter and the foot. The 

 water in this and similar cases has only access to the spaces 

 left internally between the lamince of the branchiae (oviducts 

 of Home) from behind : it is over these spaces that the secre- 

 tory organs and oviducts commonly open. We find this last 

 disposition, only with the siphons more developed, in Donax, 

 Tellina, Psammobia, and many others. There is in these 

 commonly a valve between the internal orifices ; but the wa- 

 ter is frequently ejected from both tubes. In the Mactra, Cy- 

 therea, Venus, and Venerupis, the tubes are more muscular 

 than in the Tellinidae, &c, and they are more or less united ; 

 the branchiae are in these often medianly separated. In the 

 Solen, Hyatella, Mya, Pholas, Teredo, &c, a different dispo- 

 sition takes place. Here the branchiae are prolonged into the 

 inferior siphon, and as they are not separated from the base 

 of the foot within, nor from the mantle without, the water 

 drawn in through the inferior orifice must make its exit by 

 the same, or by the anterior opening. But water is likewise 

 drawn in by the other, and so gets access to the interior in- 

 ter-laminar spaces of the branchiae ; and by this superior si- 

 phon the ova, faeces, and secretions are discharged. 1 Here 

 the branchiae are often very long, and the siphons very mus- 

 cular. We sometimes find small supplementary branchiae, as 

 in the Psammobia, Pholas, &c, or the external pair may be 

 shortened in front, as in Mya, Venerupis, and many other ge- 

 nera. In the Pandora the only appearance of the external 



1 We see this also in the Tunicata. By one orifice water enters the re- 

 spiratory sac, by the other it is drawn into the external meshes of the bran- 

 chice. The water drawn in by each opening must make its exit by the same. 

 Those writers who say the contrary must be incorrect, unless the water pass 

 through the stomach and intestine. 



