92 MR. GARNER ON THE ANATOMY OF THE 
very muscular. We sometimes find small supplementary branchie, as in the Psam- 
mobia, Pholas, &c. The external pair may be shortened in front, as in Mya, Venerupis, 
&c. In Pandora the only appearance of the external lamine consists of two very nar- 
row strips at the base of the others; this is the case also, according to de Blainville, 
in the Osteodesma, allied to Pandora. Though it is not by the action of the orifices or 
siphons, or by the relaxation of the closing muscles, and the opening of the valves, 
that the water is drawn into the mantle, yet these actions accompany the influx; and 
though the water commonly escapes in a continuous stream from the action of the cilia, 
a sudden ejection of it frequently takes place, accompanied by a closing of the valves 
and a contraction of the siphons. 
Excretory System. 
The veins of the mantle, which are very numerous, appear to secrete the valves, 
and often contain quantities of carbonate of lime, visible with the microscope in the 
form of minute spicule. In the freshwater Muscle, anatomists have been puzzled to 
account for the appearance at certain times of a greyish matter diffused over the whole 
body, and entering into all the tissues. The accumulation of this matter in the veins 
of the mantle has led to its having been considered as the male organs. We know 
that the shell is more increased at some periods than at others, and this accumulation 
may precede the deposition as a provision for its accomplishment, or it may be for the 
purpose of being thrown off by the excretory organs ; as it is about the veins which 
surround them that the accumulation principally takes place. This grey matter is 
principally carbonate of lime. | 
The excretory organs throw off mucus and colouring matter as well as carbonate of lime, 
which latter is often found in them in the form of concretions; also uric acid. In the 
Pecten a minute orifice leads directly on each side into them. The oviducts likewise 
enter them. Above, each excretory sac leads into a single transverse cavity under the 
pericardium. In the Unio; &c. an orifice close to that of the oviduct leads into a large 
cavity of the mantle under the pericardium, into which the excretory organ opens by an 
internal orifice on each side. Bojanus was not aware of this internal opening, or he 
would not have considered these organs to be lungs. The external orifice is seen to 
open at the anterior angle formed by the foot and the branchie. The oviduct is also 
distinct from the sac in Modiola, Mytilus, Lithodomus, &c., whilst in Tellina, Cardium, 
Mactra, Pholas, Mya, and most others, the ova are discharged into the excretory organs. 
Generally the orifice of the excretory organs is near the posterior muscle, and the ovi- 
duct more anterior. The former is often so minute as to be found with difficulty. In 
the Oyster the vessels do not seem to form a gland, but throw off from their extremities 
into the external meshes of the branchie. The water drawn in by each opening must make its exit by the 
same. Those who say the contrary, appear to do so erroneously, unless the water passes through the stomach 
and intestine, 
