LARVAL OR SWIMMING STAGES . 37 



cult, for, according to the biogenetic law, the younger they are the more 

 nearly they resemble some stage of the common original ancestor, and of 

 course approach one another in likeness. Under such conditions the 

 practicable, distinguishable characters may again be different and re- 

 quire a more critical scrutiny. The generic characters which distinguish 

 a full-grown mussel, clam, or oyster do not hold good for the umbo-larvse 

 of these species, and the most striking features of the umbo-stages in their 

 turn have to be relinquished when we come to consider the straight-hinge 

 larvae. 



Time of Occurrence of Oyster Larvae in Plankton. — Having reached 

 the point when the youngest shell-stages of the oyster larva, raised from 

 artificially fertilized eggs by Brooks, Rice, Ryder, Nelson, myself, and 

 perhaps by others, could not any longer be kept alive and growing, and 

 their further development could not be followed; and having determined 

 when, where, and how the succeeding stages may be procured and 

 identified ; we may now return to their description in the order of size and 

 age, following from the largest described and figured by Brooks. 



In 1904 it was not until the fourth week of July that my observations 

 became sufficiently advanced to permit a conjecture as to what particular 

 larva was the j^oung of the oyster. Fortunately I had preserved samples 

 of the plankton at short intervals from July 8th, which I could re-examine 

 after I had learned to distinguish the oyster. In this material (preserved 

 in formalin or in alcohol) oyster larvse first occur on July 11th, and rang- 

 ing from 12 to 20 units in length. 



In 1905 I had kept samples between June 7th and 26th, and on the 

 latter date there were a few oyster larva) 14, 15, and 16 units in length. 

 This is the earliest record I have of oyster larvse in the plankton. Putting 

 the two years together, these samples range from June 7th to Sept. 19th. 



In 1909 I preserved plankton every two or three days between June 

 25th and September 3rd, taken at Caraquet, Shippigan, Bay du Vin, 

 Richibucto, Buctouche, Cocagne, Shediac and Point du Chene, on the 

 east coast of New Brunswick, and at Charlottetown, Summerside, Cas- 

 cumpeque, Bideford, Grand river, the "Upper bay," Richmond bay and 

 Malpeque bay, in Prince Edward Island. Besides the occasions on which 

 I kept preservations, I examined fresh material almost every day, and 

 sometimes from several localities on the same day. In this year I was 

 prepared from the first to recognize, almost at sight, any stage of an oyster 

 larva, so that the observations made are quite correct for the locality and 

 season. I am satisfied, however, that the season was exceptionally late 

 (about three weeks later than usual), and not only for marine faunas and 

 floras, but it was a subject of concern to fishermen and farmers alike. 

 Minute oyster larvse, of which the shell measured 10 x 8:7, were first ob- 

 served in fresh material at Cocagne on July 22nd, but later careful search 

 through the preserved material showed an occasional specimen on July 



