SECT. xi. MOLLUSC A. 229 



SECTION XL 



MOLLUSCA. 



ALTHOUGH the Mollusca do not come within the limits 

 of this work they nevertheless afford objects worthy of 

 microscopic investigation. The gills of a bivalve mol- 

 lusk are like crescent- shaped leaves fixed by their stalks 

 to the transverse extremities of the mantle, so that the 

 greater part floats freely in the water. 



To the naked eye the gills appear to be formed of 

 radiating fibres of admirable structure ; but the micro- 

 scope shows that each leaf consists of a vast number of 

 straight transparent and tubular filaments, arranged side 

 by side so close that 1,500 of them might be contained 

 in the length of an inch. These filaments, however, 

 apparently so numerous, in fact consist of only one ex- 

 ceedingly long filament in each gill, bent upon itself 

 again and again throughout its whole length, both at the 

 fixed and free ends of the leaf. These long filaments 

 are fringed on both sides by lines of cilia continually 

 vibrating in contrary directions. By this action a cur- 

 rent of water is perpetually made to flow up one side of 

 the filaments and down the other, so that the blood 

 which circulates in their interior is exposed throughout 

 their long winding course to the action of oxygen in the 

 water. The duration of these vibrations in the mollusca 

 is marvellous. The cilia on a fragment of a gill put 

 into water by Mr. Gosse fifteen hours after the death of 

 the mollusk caused a wave to flow uniformly up one 



