Remarkable Distribution of Mytilus Polymorphus. 195 



meable to air than they are found in any other of the vertebratae. 

 — 7. The reservoirs for the air are not always symmetrical ; their 

 form and extent depend entirely upon the form and reciprocal 

 situation of the organs among which they are placed. All that 

 seems to be fixed is, that the total quantity of air received in 

 these pneumatic pouches, on the right side of the body, is equal 

 to that received on the left. Unless this condition were main- 

 tained, flying would be impossible, and walking would be diffi- 

 cult. — 8. No portion of the structure of birds, including even the 

 last phalanges of the wings and feet, and the last caudal vertebrae, 

 is impervious to air. The quill of the feathers makes no exception 

 to this, as has sometimes been alleged. — 9. The air within the 

 head has a distinct circulation, and does not communicate di- 

 rectly with the air-passages of the rest of the body. — 10. In no 

 part is the air in immediate contact with the viscera or the nu- 

 tritious juices ; it is invariably only through a membrane, how 

 fine and transparent soever it may be. This fact would go, by 

 analogy, to establish, that the air filling the lungs, even to the 

 last ramifications and ultimate cells, comes in no other way in 

 contact with the blood, in its ultimate vessels, than through a 

 fine and delicate membrane, as thus happens in birds. — 11. The 

 large quantities of air which birds can thus introduce into their 

 bodies, and the force with which they can again expel it, is the 

 only consideration which can explain how so small a creature as 

 the nightingale, for example, can produce notes so strong, and can 

 sing so long and so delightfully, without any apparent fatigue. 

 15. On the Mytilus Polymorphus, or the Driessena Polymor- 

 phu, a remarkable species ()f imissel. — M. Van Beneden, Conser- 

 vator of the Natural History Museum of the University of Lou- 

 vain, has presented to the Academic Royal de Sciences of Brus- 

 sels, a memoir concerning the anatomy and natural history of the 

 Driessena polymorpha, a new genus in the family of the My tilaceae. 

 Pallas was the first who, in his Voyages, discovered this mussel in 

 the different rivers of Russia, and also in the Caspian Sea. The 

 great variety of forms under which it appeared, did not escape the 

 observation of this naturalist, who designated it under the name 

 o{ Mytilus polymorphus. That a true mussel should inhabit both 

 fresh water and the sea, appeared so extraordinary to Lamarck, 

 that he did not hesitate to regard this opinion of Pallas as erro- 



