POST-LARVAL, FIXED OR SPAT STAGES 53 



less trouble and loss of time if a few extra details had been inserted, es- 

 pecially as to measurements and drawings. 



The nearest relative of the oyster that is often mistaken for a young 

 spat-oyster is Anomia (silver-shell), which ranges in size from that of a 

 silver ten-cent piece downwards. By one who has studied Anomias, 

 they can nearly always be recognized at sight and distinguished from the 

 oyster on account of their shape and surface. In cases of doubt it requires 

 but to pry them free from their support with a knife-blade and observe 

 the looser mode of attachment and the perforation in the under (right) 

 valve of the shell, through which passes a short, flexible stalk of attach- 

 ment. The oyster spat is fastened by cementing the whole or a large part 

 of the left valve solidly to the supporting rock or shell. 



Young, light-coloured Crepidulas are often difficult to distinguish, 

 but they may be slid along on their attachment or pried off, when it will 

 be observed that there is only a single shell, and in place of the lower 

 valve or a stalk there is a broad, fleshy, clinging and creeping foot. 



Colonies of Bryozoa (polyzoa) may resemble in size, colour, and shell- 

 like surface the young oyster, but are easily distinguished by observing 

 through a lens the assemblage of similar, distinct chambers (zooecia). 



Incrusting colonies of plants, such as Ralfsia, are misleading to those 

 unaccustomed to the use of a lens or a microscope. 



These and such-like other animals and plants, as well as the aggre- 

 gation of mud, silt, ooze and other matters, make it increasingly difficult 

 to discover the very youngest stages of the oyster-spat, which have in fact 

 been seldom found and by a very few specialists in the subject. 



At the time when I began my work on the oyster the microscopic 

 spat had never been studied in Canada, and there was nobody here to give 

 any first-hand information. One summer had been spent at Malpeque with- 

 out any progress towards discovering it. It was apparently a very difficult 

 subject. I at once began making observations with a view to retracing 

 from older to younger specimens, in order to learn where were the likeliest 

 places to find them, upon what objects they would be fixed, and what they 

 would look like. At the same time I began experiments with a view to 

 catching the earliest stages precipitated from the water, where presumably 

 there were free-swimming larvae present. Besides I rummaged through 

 what literature was accessible to find what others had done, and to obtain 

 suggestions. 



Of the common bivalve mollusks the adults of Ostrea, Anomia and 

 Mytilus, are fixed to objects of support. Mya, Venus, Mactra, Saxicava, 

 Clidiophora, Macoma, Ensis, Pecten, Yoldia, Mesodesma, Kellia, Tottenia, 

 and the rest, are free-living — creeping or burrowing — species, and, as such, 

 might be eliminated from consideration. Mytilus could be easily recog- 

 nized from its blue colour, shape, and byssus attachment. Any young 

 fixed-bivalve was very likely to be either an oyster or a silver-shell. But, 



