NATURAL HISTORY OF THE BAY SCALLOP 
605 
POSTLARVAL DEVELOPMENT 
It is usual to consider the late veliger or prodissoconch as ending the larval 
stage, and this is done here, although it is realized that the changes yet to be passed 
through are considerable. 
The first postveliger stage is here termed the nepionic (after Jackson) rather 
than dissoconch 6 and begins with the appearance of the compressed, wide-spread- 
ing shell characteristic of this stage, av y Sc 
not only in the scallop but also in 
the oyster. The new shell growth 
quickly assumes a shape resembling 
that of the adult, with long, straight 
hinge, byssal notch, and cycloid 
outline, but without ribs. (Figs. 2 1 
and 22.) It begins with a length of 
about 0.18 millimeter and ends with 
one of about 1 millimeter (fig. 21), 
30 0 
250 
200 
/SO 
100 
SO 
when the ribs begin to appear. 
During this period of growth gills 
attain 15 or 20 reflected filaments, 
heart and pericardium become plain- 
ly visible, the intestine comes to lie 
close to the adductor muscle which 
becomes large and differentiated 
into motor and catch portions. A 
few ocelli (6 in 1 specimen) and ten- 
tacles (15 in 1 specimen), together 
with the flap, appear around the 
mantle margin and palps near the 
mouth. The animal can attach by 
the byssus, crawl with the foot, 
swim much as in the adult, and 
float at the surface with the foot 
extended along the surface film. 
In the laboratory a specimen 
even floated for two hours at 
the film after the foot had been 
withdrawn into the shell. The 
foot is large, ciliated, and very 
active; the tentacles, papillose 
and sensitive. At or just after 
the end of the nepionic period 
(as indicated by shell develop- 
ment) the visceral mass and filaments of the outer demibranch begin to appear. 
A transition or plicate stage is recognized by Jackson. This (Fig. 21m) begins 
with the appearance of shell ribs and continues to a size of about 4 millimeters when 
an appearance strikingly like that of the adult is attained. During this period the 
“guard” tentacles appear, and lips, gills, mantle margin, visceral mass and struc- 
tures, generally, attain rather closely to the condition of the adult. 
« Dissoconch is sometimes used specifically to designate this early stage. However, Jackson (1890, p. 281), who apparently 
originated the term, applies it to the shell of the adult and of all postveliger stages. See also Korschelt (1900). 
3 00 
250 
200 
150 
too 
50 
50 
SO 
50 
50 
50 
0 
\ 
1 
JA 
N. / 
7,18 
27 
1 
i 
1 
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X- 
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i 
i 
F 
EB. 
18,1 
9 &■ 
21, 1 
927 
1 
l 
s 
V.., 
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MAR 
58. & 
53,1 
921 
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R / 
8 .JR 
z:l 
r 
*». 
J 
HA 
r / 
l/s 
21 _ 
1 
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HIM 
F 2 
921 
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JUL 
y 2 
3, / 
927 
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— 
\ 
5 10 520 5 30 540 530 5 00 5 70 5 80 5 SO 5 W0 5/0 5 
Figure 23. — Length-frequency curves based on one collection at Fivers 
Island, for each month of the first half of 1927 (5-millimeter group- 
ings). (See Table 4) 
