58. 
Shells. 
S.A. NAT., VOL. XlV. 
FEt^UARY, 1933. 
a car. 
'-Hie gills are enclosed in a fleshy bag with an inlet and outlet 
hole for the water circulation (fig. 4A). Where the shell is of a 
burrowing kind, the holes arq replaced by long tubes which pro- 
ject above the surface of the sand in communication with the 
dear water above (fig. 3B). 
1 he animal has no head and no real mouth (though primi- 
tive forms most likel)’ had), the alimentary tract being little 
more than a tube with the gullet at one end the rectum \t the 
other, and, peculiarly enough, passes through the heart (fig. 4A). 
I he shell is in two parts and on the inside of either mav -be seen, 
(fig. 4.^), the scars of two muscles which close the shell; an clas- 
tic like ligament which opens the shell as the spring opens the 
Fig. 3. .7. — The fleshy bag with inlet and outlet holes. B . — The 
long tubes of the burrowing cockle. 
lid of a snufi-box; aitd beneath this the hinge teeth tvhich inter- 
lock the two halves of the shell. It is supposed that 
most primitive cockles had no hinge teeth, later vertical ones 
formed, then horizontal, and then a combination of both. Even 
this specialised combination is not strong enough for the gigantic 
Queensland clam, which has accordingly developed an undulating 
margin opposite the hinge teeth so that the heavy shell may 
