ANATOMY AND LIFE HISTORY OF MOLLUSCA. 121 



in plaits which are rounded at the edge ; but the lobes are disunited 

 throughout, nor do they join until they reach the upper surface of 

 the posterior adductor muscle. The gills are united before 

 and behind the foot. 



In Section A of Siphonidae there are short siphons. The part 

 of the mantle which surrounds the orifices already spoken of 

 becomes united, and forms a tube which is double or divided at 

 least internally by a partition. In the lower tube the water 

 passes in, being drawn by a movement of the cilia, and passes 

 out again by the upper tube, the current removing at the same 

 time the excreta. In some cases the siphons are short and the 

 pallial line simple, that is without a deep sinus. Of this we have 

 a good example in the common Chama (sp. 1) of Port Jackson. 

 In the family Tridacnidse, which is distinguished by having the 

 adductor muscle single and nearly blended with the pedal muscle, 

 the mantle-lobe is extensively united, with however a large 

 anterior opening. There is a small grooved foot near the hinge. 

 The siphonal orifices are surrounded by a thickened pallial border. 

 This genus is well represented in Australia by the large clam 

 Tridacna gig as, L., where the size the organs offers special facility 

 for study. It is very common on all the Barrier Reef. 



To mention all the various modifications of the siphonal tube 

 which can be studied amongst our Mollusca, would exceed the 

 limits proposed by this essay. It will be sufficient to mention the 

 genera Cardium, Lucina, Cyelas, Circe, Crassatella, Cypricardia, 

 Cardita, and Venus, all of which have common species on our 

 coasts, and are typically Australian. Nearly all the genera 

 mentioned above have been examined by me, and do not offer 

 anything of special interest. In Panopcea australis, Sow., we have 

 probably the double siphons in their highest degree of development, 

 as they are largely protruding from the shell and covered with a 

 thick wrinkled skin. Living specimens are occasionally met with 

 in the Harbour, but they are rare, though the animal cannot be 

 considered uncommon, as single valves are continually found. 

 By digging for them I am convinced many would be obtained, as 

 they are gregarious. 



This seems a proper opportunity, as I am dealing with the 

 organs of respiration, to refer to the circulating fluid or the blood 

 of Australian Mollusca. The blood has formed the object of 

 special study by many naturalists. As early as 1846, Dr. T. 

 Wharton Jones read some papers before the Royal Society of 

 London on the blood corpuscle in its different phases of development 

 in the animal series. He found in the blood of the common 

 whelk (Buccinum undatum, Lam.), granule cells and nucleated cells 

 essentially similar to those of the blood of Annulosa. In Mytilus 

 edulis, L., or common mussel, similar cells were found. The blood of 



