SHELL-FISH. 
241 
short but close fringe. In the beautiful Pectens, “ the 
butterflies of the Mollusca,” the mantle is still further 
modified, for it is furnished with 
four rows of long moveable con- 
tractile tentacles, and with two 
rows of eyes that sparkle and glow 
like the most brilliant gems. 
Another and a parallel modifica- 
tion takes place in the breathing 
organs, which, instead of being a Pocton. 
closed sac, as we saw it in the Aacidice, become thin flat 
leaves, much like the folds of the mantle or the shell-valves, 
which are placed a pair on each side. Their structure is 
no less modified than their form, for instead of oval ciliated 
cells on the internal surface, each of the four leaves (in the 
Pecten, for instance) consists of a vast number of straight, 
slender, transparent filaments, evidently tubular, arranged 
side by side, so that 1500 of them would be contained 
within the length of an inch. Strictly, however, these are 
but one filament, excessively long, bent upon itself again 
and again, at both the free and the attached end of the 
gill-leaf, throughout its whole extent. This repeated fila- 
ment is armed on each of two opposite sides with a line 
of vibrating cilia, the two lines moving in contrary direc- 
tions, exactly as on the tentacles of the Polyzoa, which 
are the breathing organs there ; by this action a current 
of water is made continually to flow up and down each of 
these delicate filaments ; so that the blood which circulates 
in their interior (for they are, doubtless, blood-vessels) is 
continually exposed, throughout this its long and tortuous 
course, to the action of oxygen. 
Q 
