NATURAL HISTORY OF THE BAY SCALLOP 
575 
the valve margin. The brown coloring has been found by Belding (1910) to be 
undestroyed by acid which removes other colors from the shell. 
A thin, but definite, periostracum is present. 
MANTLE 
Morphologically the mantle is considered to be a fold of the integument of the 
dorsal portion of the molluscan body (Cooke, 1895). In the Lamellibranchia it con- 
sists typically of two equal portions or lobes which line the two valves, which they 
secrete, and surround the other soft parts and the mantle or pallial cavity. Because 
of their structure and blood supply they are held to be very important for respira- 
tion (Dakin, 1909, considered them the principal organs of respiration) or even to be 
the only important respiratory organs (Pantin, 1928). 
In Pecten the mantle lobes differ somewhat in size and shape even as the valves 
differ. The margins of the two lobes are free from the shell and, except near the 
hinge line, not united one to the other. Anteriorly the united portion is very short, 
posteriorly about ecjual to the width of the shell ear. Not only are the pallial lobes 
largely ununited, except where contiguous along the dorsum, but they are generally 
free from the inclosed soft parts, being adnate in Pecten irradians only to the adductor 
muscle, the pericardium, a portion of the surface of the digestive deverticula, the 
cephalic extremity of the branchial axis, and a portion of each of the outer labial 
palps. The internal epithelium of the mantle is ciliated. The external epithelium, 
especially that of the free margin, secretes the shell (Cooke, 1895). 
Except for the margins the mantle is very thin and transparent. Across it, 
nerves and even blood vessels may be plainly seen. The membranous structure and 
the elaborately branched vascular system indicate an important respiratory function. 
The free, marginal portion (figs. 3 and 4) of the mantle, peripheral to the pallial 
line, is thick, tough, highly pigmented, and very complex. It is supplied with an 
elaborate system of radial and concentric muscles, many tentacles, ocelli, and a large 
nerve (the circumpallial) which functions as a ganglion. Three folds of the mantle 
margin are recognized. The shell jold may be taken as extending from the pallial 
line, demarking the free margin from the fixed portion of the mantle, to the periphery 
of the shell, where the mantle is reflected inwards, and ending in the periostrachal 
groove. Dakin (1909), working with the European Pecten maximus and P. opercu- 
laris, states that it bears long tentacles. None was observed in P. irradians or 
reported by Drew (1906) for P. tenuicostatus or P. grandis (see p. 571). 
This fold is radially convoluted to form the ribs or rays of the shell. 
The middle, sensory, or ophthalmic fold extends from the periostrachal groove 
to the base of the flap or velar jold. On it (figs. 3 and 4) are the ocelli and highly 
extensible and contractile tentacles in a band several tentacles wide. These tentacles 
are smallest next to the groove and largest next to the flap. Possibly the largest or 
most extensible of all occur near the ears. On occasion they are extended to the 
surprising length of about 2 inches. The ocelli, especially the larger ones, are gener- 
ally in line with the larger tentacles, but are wanting in a section ventral to the ears. 
They vary considerably in numbers, and in adults are more numerous on the upper 
lobe. Because of the variations in numbers and size of eyes, it has been suggested 
(Drew, 1906) that new eyes are added with growth and that the number of eyes may 
be an indication of age. Drew worked with scallops not only of a different species but 
probably much longer lived. In P. irradians the quota, or very near it, for the lower 
lobe is attained early. Specimens a centimeter long possess but few less than those of 
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