158 



ANATOMY OF THE DONAX. 



putting them into my improvised 

 aquarium, and the captive donaees 

 were just a trifle thirsty. It did not 

 take them ten seconds — so quick- 

 witted are they — to learn that they 

 were at home again ; and ten seconds 

 later every man-jack of them was do- 

 ing his very best to hide. But this 

 was precisely what I did not want, 

 and had guarded against, by giving 

 them too little sand to cover them- 

 selves. Soon discovering this state 

 of affairs, the most of them grace- 

 fully gave np trying, and began to 

 open their shells where they were, 

 with what Mr. Darwin would term a 

 fine " adaptation to environment." 



Now, if you carefully open a do- 

 nax (or any similar bivalved mollusk) 

 you find just underneath the shell a 

 thin-fringed membrane or skin, which 

 clings to the inside of the shell ; this 

 is the " mantle" of the animal, cover- 

 ing him like an undergarment. Lift- 

 ing this, you will perceive two thin- 

 ner and shorter membranes — the gills 

 of one side — and underneath them a 

 sack which contains the heart and di- 

 gestive organs. Below this sack — 

 that is, opposite the hinge— is a thick, 

 yellowish, tough portion, called the 

 "foot," not because it is formed in 

 the least like the foot of a quadruped, 

 but because it serves the same pur- 

 pose of locomotion. This "foot" is 

 the hinder part of the animal, and in 

 the thin end of the wedge ; you can- 

 not speak of it as at the tail, for bi- 

 valved mollusks have no head ! With- 

 in the thick, blunt, anterior end of the 



