The Swan-Mussel and its Anatomy. 71 



janus/" concerning the function of which organ different 

 opinions have been held. It is probable that this organ, 

 which is double, receives the blood from the veins, and that 

 the greater portion of the circulating fluid finds its way thence 

 to the vessels of the branchiag, while another portion flows 

 directly to the heart. But the actual course of the blood 

 through the arteries and veins is extremely difficult to study 

 in detail; the main facts, however, as we have stated them, 

 may, we think, be depended upon. It will be seen that one 

 pulse of the aortic ventricle is sufficient to impel the blood 

 through the two systems of vessels and to bring it again to 

 the triangular auricles. 



The orgaus of respiration consist of the two pairs of gills 

 already mentioned; each gill is formed of two membranes, 

 connected together, in the case of the exterior pair, by several 

 transverse septa, forming a number of divisions or pockets (see 

 Fig. 5), which are filled, at certain seasons of the year, with 

 thousands of young fry. Numerous blood-vessels are observed 

 to traverse the substance of the gills, which, being provided 

 with many cilia, serve by their constant action to oxygenate 

 the blood. 



The Anodonia, like the rest of the TTnionidce, moves by 

 means of its long fleshy tongue-shaped foot, which it extends 

 forwards to a considerable length upon the mud, and then 

 making that point a lever, draws itself onwards ; its track may 

 often be seen in the mud ; the movement is very slow. 



If an Anodon be taken out of the water and its valves 

 opened, a quantity of water is seen to issue from the muscular 

 foot. According to the experiments of two recent observers, 

 these animals are enabled thus to distend their foot with water, 

 so that it appears almost transparent, by means of a number 

 of aquiferous canals whieh traverse the foot, and are every- 

 where in apposition with the blood-vessels. The water is 

 drawn into the pericardial space, when the bivalve opens its 

 shell, and from thence to the interior of the blood-vessels ; 

 from the blood-vessels these authors suppose it to transude 

 into the system of water-tubes, and to find its exit at a num- 

 ber of pores on either side of the foot of the animal. Repeated 

 dissections of these animals have failed to reveal to my own 

 eyes the presence of these foot-pores. Both M. H. Meckel 

 and M. Moquin-Tandon have been equally unsuccessful ; but 

 the careful experiments of Dr. Rolleston and M. Robertson 

 seem decisive in favour of their existence.* 



With regard to the organs of generation, much remains as 

 yet t unexplained. The ascertained facts may be thus briefly 



* See Annals and Magazine of Natural History for December, 1862. 



