88 



ANIMAL BIOLOGY 



sluggish, sand-burrowing creatures; the Shipworms riddle wharves 

 with their tunnels, while the Scallops swim about by rapidly open- 

 ing and clapping together the valves of the shell. (Fig. 49.) 



The most distinctive feature of the Bivalves, aside from the 

 shell, is their peculiar method of securing food from a current of 

 water that is kept in motion by the activity of cilia on the gills, 

 mantle surface, and about the mouth. In brief, water laden with 

 oxygen and microscopic animals and plants is drawn through an 

 opening, the inhalent siphon, into the mantle cavity where the 



Pericardial sac 



Umbo 



Anterior adductor 

 muscles of foot 



Labial palp 

 Mouth 



Anterior 



adductor 



muscle 



Ventricle 

 Auricle 

 Rectum 

 Kidney 



Bulbus arteriosus 



Posterior adductor 

 of foot 



Upper branchial 

 chamber 



Gill 



Posterior 

 adductor 

 muscle 

 — Anus 



Exhalent 

 siphon 



Inhalent 

 siphon 



Foot 

 Mantle 



Liver 



Gonad 

 Intestine 



Lower branchial 

 chamber 



Fig. 49. - - Hard-shell Clam, or Quahaug, Venus mercenaria. Dissection 

 showing structures visible when left valve, mantle, and gills are removed. 



sieve-like gills are suspended. Passing through the gills the food is 

 picked out and carried by ciliary action to the mouth, while at the 

 same time the blood in the gills is aerated. From here the water 

 current passes on out by the exhalent siphon, carrying with it 

 various waste products. 



The reproductive process varies considerably in different Bi- 

 valves. Oysters spawn in the spring, liberating the sperm and 

 eggs, and the zygotes develop into microscopic free-swimming 

 larvae. Within a week these sink to the bottom and become at- 



