No. 522] THE CANADIAN OY, 



2) there are about sixteen long filaments on the left side 

 and about ten short filaments on the right side— the left 

 gill extending in front of and behind the right one and 

 occupying most of the gill-chamber (Figs. 4 8). In spat 

 of 2.5 mm. height there appears on the right side, out- 

 wards from the already present gill, a third series of 

 minute, papilla-like filaments (Fig. 14), and when the 

 spat reaches :\ mm. in height a fourth series is to be seen 

 in the corresponding position on the left side (Fig. 15). 

 They increase in size during the growth of the spat 

 until the animal possesses four complete gill-leaves cor- 

 responding with those of the adult. 



During larval development the gills of the oyster make 

 but little progress towards the complicated structure of 

 the adult. The free- swimming animal being small, res- 

 piration can for a time be partly subserved by its surface. 

 But fixation effects a marked change in its mode of life 

 and is followed by far-reaching modifications in its or- 

 ganization. Rapid increase of size demands improved 

 facilities for respiration. But, as the animal no longer 

 comes in contact with its food through swimming move- 

 ments, it nmsl depend upon the respiratory currents for 

 bringing food to itself ; hence the gills acquire a double 

 importance. Moreover, since the conditions favorable 

 to bilateral symmetry are interfered with, the equal 

 balancing of right and left sides in the further growth 

 of organs must be left to heredity. At the time of fixa- 

 tion, as we have seen, the gills are represented by two 

 bilaterally symmetrical inner gill-leaves. The left (now 

 under one) grows much faster than the right (now upper 

 one) so that there is more room and less pressure above, 

 facilitating the development of the right outer gill-leaf 

 before the corresponding one of the left side. Irregu- 

 larity also soon becomes noticeable in the higher level 

 of origin of those of the right side and in a tendency 

 towards a radial symmetry of the organs with reference 

 to the posterior adductor muscle. 



A study of sections of larvae and young spat reveals 



