358 



THE AMERICAN NATURALIST [Vol.XLIV 



The foot, after fixation of the larva, no doubt ceases 

 to grow, perhaps becomes reduced, but in any case 

 shrinks against, is absorbed into and incorporated with 

 the anterior ventral wall of the rapidly enlarging ab- 

 domen. In 1 mm. spat it is scarcely recognizable as a 

 median, grooved, muscular thickening between the lower 

 lip and the first gill-filaments. 



Jackson (12, p. 303) : "The fact that a velum, or swimming organ, 

 exists up to the period of permanent fixation accounts for the great 

 reduction of the foot, because that organ is unnecessary while the 

 animal is provided with another locomotive organ, and is useless for 

 progression after the animal is attached. The reduction of the foot 

 is clearly attributable to disuse and a high degree of concentration of 

 development . . . seen so markedly in the development of the oyster." 



part designated " foot " by Jackson and others is not the real foot, 

 which is only to be found at a very much later period and which they 

 had not recognized. 



The intestinal system lias all its parts represented in 

 the larva. With the growth of the spat these surfer cer- 

 tain alterations in relative sizes, shapes, and positions. 

 Perhaps the most radical change is produced by a rota- 

 tion of the body in such a way that the mouth moves for- 

 wards and upwards towards the antero-dorsal margin 

 of the prodissoconch. This doubtless accompanies and 

 is associated with the loss of the velum, which in the 

 larva is so large as to occupy all the fore part of the 

 cavity of the shell, forcing the mouth and oesophagus 

 backwards to near the median frontal plane of the body. 

 In spat under 1 mm. in height when the velum is com- 

 pletely cleared away, the mouth can occupy its normal 

 position as in the adult. Such a rotation may appear 

 at first thought inexplicable, but when it is remembered 

 that in the prodissoconch the body of the larva is pos- 

 sessed of great freedom of movement, being at times 

 thrust far forwards, putting the retractor muscles on the 

 stretch, it can be readily understood. In fact it is con- 

 ceivable that these muscles may be made to do duty in 

 producing the rotation and in fixing the body in its new 



