ANATOMY AND LIFE HISTORY OF MOLLUSCA. 181 



unequal segmentation, and the small segments gradually envelope 

 the large hypoblast spheres. Then one side of the embryo is 

 flattened and it becomes covered with short cilia, which causes it 

 to rotate within the egg-capsule. The shell subsequently appears 

 as a double organ. The two valves grow very fast and cover the 

 larva, in which mantle-flaps begin to be perceived. This is the 

 stage when the ovum is hatched, and the young are carried down 

 by the currents into the gill-chamber, where they become 

 developed and the shell rapidly forms so as completely to cover 

 them. The time when the nursing of the mother ceases I have 

 been unable to ascertain, but I believe that it is sufficiently 

 developed to be able to affix itself as " spat." 



These observations may have a certain value, for it points to 

 the value of the mother-oysters as nurses, which may influence 

 the methods of preservation. In any case it will be interesting 

 to observe that young oysters are reared in the gill-chambers of 

 the mother, in the case of the Australian oyster (Ostrea mordax, 

 Gould). Of course this does not affect the question of the 

 American oyster. In the case of that species it has been over 

 and over again affirmed that the American oysters rear their 

 young in the gill-chambers like the freshwater mussels. This on 

 the other hand has been over and over again denied by high 

 authorities. The question cannot yet be said to be set at rest. 



Now with regard to Unio the question is one of great interest, 

 for we have many widely distributed species in Australia, all 

 differing in only the slightest possible details from European or 

 American species. The peculiar interest which attaches to the 

 freshwater mussel, however, is this — that the eggs are retained 

 within the gill-chamber of the female mussel through all the 

 earlier stages of development. When the young mussel escapes 

 from the egg, moreover, it is so unlike a mussel that its relationship 

 was never suspected. It has hooks to attach itself, which it does 

 promptly, to the tails of fishes, where it was found and thought 

 to be a parasite, and named Glochidium. This larva possesses a 

 bivalve shell on which are the hooks above mentioned. The 

 mantle can also be discerned, and an adductor muscle for closing 

 the shell when required. When the larva is swept out from the 

 body of the parent, and becomes anchored as already stated, it 

 grows rapidly, and as it does so becomes more and more like the 

 well-known freshwater mussel. 



The various changes to which the mussel ova are subject have 

 been observed and described by many different microscopists, 

 amongst whom there is still considerable difference of opinion. 

 Without attempting to follow these technical details, one or two 

 points may be given as throwing light on what has to be observed 

 in Australia. The development of the velum from the ovum 



