ACEPHALA. 259 



world with the mantle-chamber. When these siphons 

 become greatly developed there are connected with them 

 strong retractor muscles, to 

 draw them back at times of 

 danger, etc. All of these 

 muscles adductors, retract- 

 ors, etc. leave their impress 

 on the shell, so that the stu- 

 dent, with the shell alone, 

 knows of some of the struc- 



of thp soft mrts Fl ^' 1 ^ 8 ~~ Insid of . Wvalve shell 



1 Wie bOlt pdl Lb. showing muscular impressions, rt, 



ia rlrnwn infn tVin anterior adductor ; p, posterior ad- 

 IS arawn into ine ductor ; ,x, siphonal muscle. 



mantle-cavity by means of 



very minute hair-like structures (cilia) which cover the 

 gills and other parts. These cilia are in constant motion,* 

 and thus currents of water are produced, flowing always in 

 one direction. This water brings oxygen to the gills and, 

 through them, to the blood. It also brings minute animals 

 and plants. These are passed on to the labial palpi, which 

 are similarly covered with cilia, and from these organs the 

 cilia force the food into the mouth. 



In the nervous system we always find cerebral, pedal, and 

 visceral ganglia, the first being above, the others beside 

 or below, the alimentary canal. Ears are present, connected 

 with the pedal ganglia; and eyes may be present, either 

 upon the edges of the mantle or at the tips of the siphons. 



The alimentary canal is always provided with stomach 

 and liver. Connected with the stomach a blind sac fre- 

 quently occurs, and in this there may be a peculiar trans- 

 parent rod, the crystalline style, of uncertain use. The 



* The teacher should demonstrate this ciliary action under the 

 compound microscope. 



