SURVEY OF INVERTEBRATES 



89 



tached to whatever object they happen to touch and, if fortunate, 

 gradually develop into adult Oysters. Fortunate, because it is esti- 

 mated that a larva has less than one chance in a million of surviv- 

 ing to attain maturity and the qualities that appeal to the human 

 palate. But one female may contain nearly half a billion eggs. 



The eggs of the Fresh-water Clams, or Mussels, are fertilized 

 by sperm entering the mantle cavity with the food current, and 

 the larvae develop in the gills which act as temporary brood- 



Dorsal 



•Fin 





'■■■"\y4 



Anterior 



ol mantla 

 Eye- 



Arm 



Long arm. 

 with suckers 



Ventral 



Posterior 



.Edge of mantle 

 -Siphon 

 •Head 



-Aim. 



Fig. 50. — A, B, Octopus, at rest and in motion, f, siphon. C, Squid, side 

 view. (From Hegner, after Merculiano and Williams.) 



pouches. Eventually as tiny clams, known as glochidia, they 

 escape, settle on the bottom of the pond or river, and die unless 

 a Fish rubs against them. In this event each glochidium becomes 

 attached to the fish and as a parasite obtains free food and trans- 

 portation for several weeks until it has developed sufficiently to 

 shift for itself. 



The economic importance of the Bivalves hardly need be men- 

 tioned. Oyster-farms in America alone produce an annual crop 

 valued at many millions of dollars. Fresh-water Mussels are the 

 basis of the pearl button industry of the Mississippi Valley. And, 



