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- ^SijjBl&sfc. 



closed sac, as we saw it in the Ascidice, become thin fiat 

 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. rt\ 



.nother and a parallel modifica- 

 tion takes place in the breathing 

 organs, which, instead of being a 



tractile tentacles, and with two 

 rows of eyes that sparkle and glow 

 like the most brilliant gems. 



Pecten. 



