ORGANS OF THE SPAT 67 



Adductor Muscles, at the time of fixation (Plate V, figs. 30-32)) are of 

 nearly equal size, the anterior being perhaps a little larger than the 

 posterior. But from this time onwards their history is different. The 

 anterior adductor becomes smaller, is moved upwards and backwards 

 from its original position, and finally it is crowded to the edge and dis- 

 appears. In my sections distinct transverse fibres can only be traced as 

 far as to the spat of 72 units ( = £ mm.) The posterior adductor, on the 

 other hand, increases regularly in size and moves downwards and slightly 

 backwards, leaving distinct lines on the inside of the shell to indicate its 

 change of position (Plate V, fig. 34). In the newly attached spat it is 

 situated in the prodissoconch, just below the hollow which separates the 

 umbos from the projecting posterior end of the shell. In spat of >5 mm. it 

 is half way between this and the lower edge of the prodissoconch. Spat 

 of -85 mm. show it moved on to the dissoconch just below the edge of the 

 prodissoconch, and those of 1.5 mm. have it the depth of the pro- 

 dissoconch, below the lower edge of the latter. In all stages it is about 

 half way between the dorsal and ventral edges of the shell, and slightly 

 behind the middle in the antero-posterior direction. The movement may 

 be effected by a slow creeping of the muscle, perhaps due to downward 

 pressure from the growth of the body above, or by the addition of new 

 fibres below and the absorption of old from above; while the impressions 

 on the shell ma}' result from the inability of the surface of the mantle to 

 deposit new layers of pearly matter under the attached ends of the muscle. 



Huxley ('83 p. 112) believed that the adductor muscle of the straight-hinge larva 

 could not be the same as that which exists in the adult, since it lies in the fore part of 

 the body and on the dorsal side of the alimentary canal while the great muscle oi the 

 adult lies on the ventral side of the alimentary canal, and in the hinder part of the body. 



Jackson (1888) discovered in his young spat that there are two adductor muscles 

 —the anterior adductor as seen by Huxley, and another, the posterior adductor — 

 the same as occurs in adult dimyarian Pelecypoda. The posterior adductor persists 

 as the great adductor of the adult oyster. 



Gills, at fixation, are present in essentially the same condition as al- 

 ready described for the full-grown larva (Plate V, figs. 30-32). There 

 are two — one on each side — attached laterally between the foot and the 

 mantle (Plate VI, figs. 4, 5, 13, 14). Each may be compared to a comb 

 having its back (the attached axis of the gill) turned upwards and its teeth 

 (the gill filaments) turned downwards. The axes slant downwards and 

 backwards past the foot, joining behind (Plate VI, figs. 7, 22), and the 

 filaments in this region are short and incompletely separated. In sections 

 of the spat there is a noticeable difference between right and left gills — the 

 left being largest and most advanced in development (Plate VII, fig. 4). 

 New filaments begin as mere knobs partially split from the posterior mass, 

 the splitting proceeding from the outside and from below but never com- 

 plete above, where all the filaments remain connected in the axis. In the 

 larva, immediately before, and in the spat, immediately after fixation. 



