POST-LARVAL, FIXED OR SPAT STACxES 57 



male Oyster is black-sick, having a black substance in the fin; the female white sick (as 

 they term it), having a milky substance in the fin." (Report U.S. Fish Com. 1892, 

 p. 307). 



Huxley (1883, p. o3) : "But sooner or later, they settle down, fix themselves by 

 one side to any solid body, and rapidly take on the characters of minute oysters, which 

 have the appearance of flattened disks, 1/20 of an inch, more or less, in diameter; they 

 are therefore perfectly visible, as white dots, on the surface of the substance to which 

 they adhere. In this condition, the name of ' spat ' is also applied to them. 



" It is unfortunate that the same word ' spat ' should be applied to things so dif- 

 ferent in their nature, as the eggs and unhatched young of the oyster, contained with- 

 in the mantle cavity, on the one hand, and the young fixed oysters, on the other; while 

 there is no familiar name for the very important stage of development which lies be- 

 tween these two. 'Brood,' 'fry,' and 'spat' would be very convenient names for the 

 three stages, if 'brood' were not already in use for the smallest of the young fixed 

 oysters. Perhaps the most convenient course will be to use 'fry' for the eggs or 

 embryos which are contained within the mantle cavity of the parent; 'larvae' for the 

 locomotive stage; and 'spat' for the fixed condition." 



Horst (1884, p. 910): "Fig. 19. Little oyster, about 7 days old; the height of the 

 primary homogeneous shell is 0-24 mm., that of the secondary part, composed of 

 prisms, is 0- 15 mm." The complete height would be 0-39 mm., which is much smaller 

 than Huxley's measurement of about 1/20 inch (=1-25 mm.) In Horst's figure the 

 larval shell measures -34 mm. in length, which will give (-34 x 15-^20 = -25) as a 

 real length -25 mm. or about f that of the corresponding American shell. Taking 

 Horst's (1882, '83, p. 386, Fig. 2) "Largest straight-hinge stage of the European oyster," 

 of which the figure measure is 19 mm. and magnification 96, we would get its real size 

 as 19 h- 96 = -198 mm. Again taking Huxley's (1883, p. 54), statement of the size 

 of the egg, about 1/250 inch in diameter, and Brooks' (1880, p. 13) measurement, about 

 1/500 inch in diameter, and we can make an approximate comparison of three stages 

 of the European and American oysters: 



European . 

 American . 



In which it appears that our oyster begins in an egg of only half the diameter, or 

 one-eighth the volume, of the egg of the European oyster (Ryder 1882, '84, p. 791), 

 but that ours develops to a larva one and a half times as large as theirs before be- 

 coming fixed as spat. The European oyster begins life with a great stock of food-yolk, 

 is nurtured and protected by the parent while it is developing to nearly double the 

 diameter of the egg, and only increases less than half this again during a short free- 

 swimming period. Our oyster begins as a free egg of only half the diameter of the 

 European, has a smaller stock of stored yolk, develops rapidly to the swimming stage, 

 takes its own chances, begins feeding early, and continues its free and independent 

 existence until it is one and a half times the length of the corresponding stage of the 

 European larva. 



Ryder (1882, '83, p. 387, Fig. 3) gives a figure of the youngest stage of a spat, 

 " having just become firmly fixed." It is reversed so that it appears to show the left 

 side. The figure measures 29 mm. and the magnification is 96 times, so that the spat 

 itself must have been 29 h- 96 = -3 mm. in length and height. I have already dis- 

 cussed Ryder's views of this stage. 



Jackson (1890, Figs. 1, 2) gives figures showing the structure of the youngest stage 

 of a spat caught on a glass slide. The real size of this spat would be 37 ■*■ 120 =• 3 mm. 

 It represents the beginning of Jackson's own observations, and is important as showing 

 the anterior and posterior adductor muscles, the velum persisting although reduced, 

 the gills, the anus, and what Jackson took for the palps, but in reality the foot some- 

 what shrunken and wrinkled. 



Succeeding stages are shown in Ryder's Figs. 4, 5, 6, 7, (representing spat of 

 about -34, -38, -51, -54, mm.) Jackson's Fig. 3 was about -55 mm. high, and his 

 Figs. 20, 21, about -91 mm. Then come Ryder's Fig. 8, about 1-6 mm., his (1882, 

 '84, p. 782) Fig. 2 of 3 mm., and Jackson's Fig. 4 of 5 mm. 



