200 
Part III. — Eighth Annual Report 
V.— ON THE DEVELOPMENT OF THE COMMON SCALLOP 
(Pecten opereularis, L.) By J. H. Fullarton, M.A., D.lSc. 
(Plates V.-YIII.) 
The Common Scallop — Pecten opereularis, L., exists in great abundance 
ia the Firth of Forth. The beds there, supplying annually to the fisher- 
men of the neighbouring shores above 1000 tons, which are entirely used 
for bait, have already been described by the writer.* 
During the summer of 1889 the Fishery Board for Scotland instructed 
me to proceed to Cockenzie, a fishing village on the shores of the F'orth, to 
study the development and life-history of the Scallop or Clam as it is 
known locally, and that was undertaken so far as the appliances at my 
command permitted. 
Nothing, so far as I am aware, has been done in the investigation of the 
life-history of the developing embryos of the Common Scallop, though 
Lacaze-Duthiers has figured the ova and spematozoa of Pecten glaber and 
P. varius. Before, however, dealing with embryological details, there arc 
certain points connected with the sexual organs which should be 
mentioned. Like the Monomyarian oyster the genus Pecten is herma- 
phrodite. The sexual organs of P. opereularis (pi. viii. fig. 34, ov. and /.), 
chiefly lie posterior to the rudimentary foot. They form a prominent 
abdominal mass stretching to the middle of the under surface of the single 
large adductor muscle. Relatively to the adductor they are situated anteriorly 
aud inferiorly ; the anterior end lies on the under surface of the liver, close 
behind the mouth. Slightly constricted behind the foot they gradually 
increase in size posteriorly till they attain their greatest breadth nearly 
opposite the posterior end of the organ of Bojanus (k). From this they 
taper quickly, and end in a blunted point. Their ventral outline is 
markedly convex, while the dorsal boundary, where this is separable from 
the subjacent tissues, is concave. The hinder pointed end of the organ is 
free, but half the distance from the posterior end of the organ of Bojanus it is 
connected to the tissue that covers the undersurface of the adductor and the 
large parieto-spjanchnic ganglion. From its posterior point of connection 
forwards, it is in a more or less close union with the subjacent organs and 
tissues. Where it lies on the adductor muscle, it is easily separated from 
the muscle, but anterior to this it is in close union with the adjoining 
tissues. It is interrupted by the foot, and, internally, by the coils of the 
intestine. The greater portion of the organ posterior to the foot consists 
of generative tissue, but anterior to the foot it simply forms a thin covering 
of tissue on other organs. 
The ovigerous area (oy.)of the hermaphrodite organ is atthe hinder blunted 
end, and its colour marks its anterior boundary from the male portion (/.) 
The bounding surface line between the two kinds of tissue is very irregular 
in different individuals. In some examples the ovarian tissue comes 
further forward, and overspreads the spermatic tissue, while in others, as we 
cut into the organ, it is seen that the ovarian tissue is as far forward in 
the deeper layers as it is on the surface. Generally the ovary occupies 
about one-third of the mass, and the lobes of the ovarian tissue dove-tail 
into the lobes of the testes, so that the surfaces of contact are very uneven, 
forming in fact, a series of pits and prominences. The intestine sometimes 
winds into the ovarian mass, but it is for the most part confined to 
that part of the organ which functions as testes. The latter is interrupted 
by the foot, but otherwise it covers the whole of the remaining surface 
from ovary forwards to near the mouth. 
* Seventh Annual Report of the Fisherxj Board for Scotland, Part III. p. 341. 
