47 



CHAPTER VI. 



OF THE PHOLAS. GENUS III. 



PHOLAS. 



"Animal an Ascidia. Shell bivalve, spreading, with several less and diffe- 

 rently shaped accessory valves at the hinge ; hinge recurved, connected 

 by a cartilage." — Linn. 



Phis animal, which Liuuaeus calls an Ascidia, is scarcely any 

 thing more than a fleshy membranaceous bag, nearly of the 

 length of the shell, apparently open at each end ; through the 

 upper end there protrudes a cylindrical muscular tube, divided 

 by a partition into two, toward the extremity ; and at or 

 through the lower end of the bag proceeds a short obtusely 

 conical foot. The shell, which is somewhat cylindrical, con- 

 sists of two valves, open or gaping at each end (p/. 1. /. 9), 

 connected together by a cartilage, which serves for a hinge ; 

 at this part the edge of the shell is reflected or turned back, 

 and in some species forms several small cells (p/. 1. /. 7. yl), 

 which are generally covered with aj smaU bony plate; there 

 are likewise one or two more small plates covering the cartilage 

 of the hinge (p/. 1./. 8) ; all these are called accessory valves, 

 the number and form varying in the different species, but 

 commonly from two to four. 



The outer surface of the shell is rough, with sharp ridges or 

 points something like a file, which roughness is supposed to 



