282 



CIVIC BIOLOGY 



species that do not have hooks are taken in with the breathing currents 

 of fishes and clamp on to the gill filaments. The tissues of the fish grow 

 over the giochidia, and within the sac thus formed they grow and 

 change into the adult form. Finally, at the end of from two to ten 

 weeks, according to their species and the temperature of the water, 



they kick themselves out of these 

 cysts and begin their free life on- 

 the bottom. So far as we know, this 

 is the only way a young fresh-water 

 mussel can be carried over this criti- 

 cal stage from giochidium to adult, 

 and this means that extermination 

 of fishes must result in extermina- 

 tion of mussels as well. 



Problems. Ascertain from the 

 nearest markets which species of 

 mussels produce the most valuable 

 shells, and make a collection of these 

 for the school museum. 



Examine specimens of valuable 

 species and make a table showing 

 the months when the gills contain 

 giochidia. Discuss the advisability 

 of a closed season including these 

 months. Estimate the number of 

 giochidia per adult mussel. 



Examine all fishes caught for 

 giochidia in gills or fins and skin. 

 How many may a fish carry? 



Try, possibly with the help of 

 the state fish commission or the 

 United States Bureau of Fisheries, 



to make a plan for the best possible utilization of streams, ponds, and 

 lakes in the locality, for both mussel and fish culture. 



Gastropods. Comparatively slight civic values attach to this group. 

 The abalones are of interest in California, and the periwinkle (Littorina), 

 brought to the Atlantic coast from Europe, where it is used for food, has 

 become abundant from New England southward. It is also of value in 

 cleaning oyster beds of seaweeds. A number of other marine forms, the 

 oyster drill ( Urosalpinx), Fulgur, and Natica, feed upon oysters and clams. 



FIG. 129. Garden slugs spinning 

 mucous threads 



Photograph by the author 



