188 INVERTEBRATE ZOOLOGY 



The Gills and Palpi. — The mantle lobes overlie the gills which 

 are typically paired structures on each side of the body. In the 

 most primitive Acephala the gills are featherlike structures 

 similar to those found in the Gastropoda. Gills of all of the 

 higher Acephala are derivable from this simple condition. As the 

 flattened filaments elongate, the distal ends of the filaments 

 become recurved and thus each filament becomes V-shaped. In 

 the sea mussels the gill filaments may be either independent or 

 united by interlocking cilia, while in the fresh-water bivalves and 

 the marine clams the walls of adjoining filaments become grown 

 together to form a continuous sheet. 



Cilia covering these gills bring water currents through the 

 siphon into the mantle chamber thereby making respiration 

 possible and at the same time bringing into the mantle cavity 

 microorganisms and other organic matter which finally enter the 

 mouth and serve as food. A pair of small flaplike structures, 

 the labial palpi, surround the mouth opening and aid in the 

 selection of food. 



The Foot. — The foot is highly characteristic in the Acephala, 

 though in some instances, as in the oyster, it has become degener- 

 ate. In the adult stage the oyster loses all power of locomotion, 

 for the left valve of the shell becomes permanently attached to a 

 rock or some other object. Among the scallops (Pecten) the 

 foot is degenerate, but locomotion is accomplished by a rapid 

 snapping movement of the shells produced by the single, heavy 

 adductor muscle. The foot is, typically, a hatchet-shaped 

 muscular organ which, by protrusion through the gaping valves 

 of the shell, becomes fixed in the sand or mud, then by contrac- 

 tion of the muscles the animal is drawn a slight distance for- 

 ward. Posterior to the foot, there frequently occurs a byssus 

 gland secretions from which form heavy silken threads by 

 means of which permanent or temporary attachment is 

 accomplished. 



The visceral mass is a softer body, lying dorsal to the foot, 

 within which various organs are located. As the name indicates, 

 there is no head in the Acephala. The digestive system has its 

 beginning in a mouth at the anterior extremity of the body, 

 located between the small leaflike labial palpi. In the visceral 

 mass, the digestive tube makes a number of coils and in most 

 Acephala terminates in a rectum which passes through the peri- 

 cardium and also through the ventricle. Gonads and liver 



