ANATOMY AND LIFE HISTORY OF MOLLUSCA. 115 



localized. This does not always happen in similar portions of the 

 animal, nor can we regard all the organs which appear to be gills 

 as morphologically identical (Gegenbaur). Moreover, instead of 

 a specialised gill, a modification of the organ of respiration may 

 be found in a respiratory canal system, which is developed in the 

 walls of the mantle cavity. In some Gastropods, according to 

 the author just quoted, this network of canals extends beyond 

 the gills into the neighboring parts of the branchial cavity, which 

 are thereby enabled to take part in the respiratory function. In 

 this way the mantle cavity is adapted to taking in air and becomes 

 a lung. An organ of this kind is found in various forms in very 

 different families oi Mollusca. It enables the animal to breathe 

 both air and water. The following observations on this subject, 

 as illustrated in an Australian Mollusk, were made by me in 

 Tasmania. The species referred to is Siphonaria denticulate^ 

 Q. and G. The shell is irregularly oval, with a protuberance on 

 the siphonal side, with 40 to 50 fine ribs of lighter and darker 

 colors. Animal dull brown, with numerous small light spots of 

 varying size ; foot yellowish, shading to orange near the head ; 

 mantle brown, fringed at the edge with whitish and black spots. 

 When the mantle is contracted the black spots seem to be the 

 points where it is drawn in. Head, a large and many-lobed mass, 

 forming a cup-like expansion round the very small mouth ; no 

 eyes visible, and, though they are represented in Messrs. Quoy and 

 Gaimard's figures of S. diemenensis, Quoy, I have never been 

 able to detect anything but a single black dot of varying position 

 on one of the lobes of the head. Above the foot on the left side 

 of the animal is a lobe which forms a kind of semi-circular tube, 

 closely pressed to the shell, and here the mantle is not visible. 

 This tube is the siphon, and is lobed so as to be capable of a kind 

 of bipartition which probably divides the orifice into an excretory 

 as well as respiratory duct. This lobe of the foot acts as a kind 

 of operculum, closing the orifice when necessary. If placed in 

 the open air the siphon tube opens at once, and it is always open 

 when the animal is taken from the rocks which it inhabits, and 

 which are never long covered by the tide. On placing weak 

 carbonate of ammonia about an inch from the orifice, the animal 

 emitted bubbles of air and showed signs of distress by movement 

 and by pouring forth water from the mantle. On immersing in 

 water animals long exposed to the air, many bubbles of air rapidly 

 escaped, and the siphon became tranquil and full of water. In 

 this state the animal continued many days. Carmine dropped 

 into the water, gradually spread out, and was drawn almost 

 imperceptibly into long threads or currents towards the siphon, 

 and then much diluted and in fine streaks. From these facts we 

 may conclude that respiration is accomplished by no muscular 



