68 COMMISSION OF CONSERVATION 



there are about eight filaments, but when the spat reaches .86 mm. in 

 height there are about sixteen comparatively long filaments on the left 

 side and about ten shorter filaments on the right side — the left gill extend- 

 ing in front of and behind the right one and occupying most of the bran- 

 chial chamber. In a spat of 1.5 mm. height I counted twenty-three fila- 

 ments and in one of 3 mm. fifty filaments in the left gill. In spat of 2.5 mm. 

 height there appears on the right side (Plate VII, fig. 14), outwards from 

 the already present gill, a third series of minute, papilla-like filaments, and 

 when the spat reaches 3 mm. in height a fourth series (Plate VII, fig. 15) 

 is to be seen in the corresponding position on the left side. They increase 

 in size during the growth of the spat until the animal possesses four com- 

 plete gill-leaves, corresponding with those of the adult — two inner, which 

 began to develop in the larva, and two outer, which did not originate until 

 after entering on the spat stages. 



During larval development the gills make but little progress towards 

 the complicated structures they possess in the adult. The free-swimming 

 larva being small, respiration can for a time be largely subserved by the 

 surface, of which velum, foot, mantle and gills form a large part. But 

 fixation effects a marked change in the mode of living and is followed by 

 far-reaching modifications in the organization of the spat. The animal 

 economizes by discarding such active muscular and sensory organs as the 

 velum and the foot, but finds some disadvantage in the matter of aeration, 

 resulting from the loss of movement and of surface. Both of these dis- 

 advantages are met by the increased surface and ciliation of the gills. 

 Moreover, as the animal no longer comes in contact with its food through 

 swimming movements, it must depend upon the respiratory currents for 

 bringing food to itself. Hence the gills acquire the double importance of 

 acting as the chief organ of respiration and at the same time of serving 

 as a gatherer of food. Under this simple arrangement the grovv^th of the 

 organism is rapid and its development hastened to completion. At the 

 same time, since the conditions favourable to bilateral symmetry are 

 interfered with at fixation, the equal balancing of right and left sides in 

 the further growth of organs must be left to heredity. But gravitation, 

 i.e. weight and pressure, acting unequally upon the two sides, soon effects 

 a marked difference; the left, now under, gill 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 corre- 

 sponding one of the left side. Irregularity 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 these organs with reference to the posterior 

 adductor muscle. 



In the youngest stages of the spat transverse sections of the filaments 

 show each of the larger ones to be flattened by pressure antero-posteriorly 

 and extended laterally, sometimes rather wedge-shaped, with the broad 



