ENSIS DIRECTUS. 105 



ENSIS DIRECTUS. (Razor-shell Clam.) 



This species is another representative of the order Eulamel- 

 libranchia and is introduced because of its adaptation for a 

 burrowing habit, and because of the great ease with which its 

 nervous system can be studied. Individuals are not uncommon 

 on mud- or sand-flats from which the water flows at low tide. 

 They may sometimes be seen protruding above the surface of 

 the mud, but are hard to approach because of their great sen- 

 sitiveness. Upon being disturbed they quickly disappear be- 

 neath the surface of the mud. 



1. Notice the shape of the shell, the way it gapes at both 

 ends, and the way the lobes of the mantle are fused. 



2. With a pencil-point or seeker stroke the tentacles around 

 the ventral mantle opening, and around the siphon openings, 

 while the animal is being held anterior end downward. This 

 will cause it to perform the burrowing movements. Study the 

 movements carefully and see what the effects would be were they 

 performed in mud. Thrust the anterior end of the shell in mud 

 and watch the result of the movements.. 



3. Water is ejected by the sides of the foot to aid in burrow- 

 ing or to enable the animal to swim, but observations on its 

 method of ejecting it are not easily made, and are sure to take 

 much time. Notice the way the anterior margins of the lobes 

 of the mantle scrape mud from the foot w^hen the foot is being 

 withdrawn. 



4. With a scalpel separate the united margins of the mantle 

 throughout their length. Slowly pry the valves apart, lift up 

 the free end of the foot and pull it posteriorly. 



The cerebral ganglia are plainly visible without further cutting. 

 They lie just posterior to the anterior adductor muscle and in front 

 of the mouth, and are widely separated. They are connected by 

 a narrow commissure, and each gives rise to a cerebro-visceral 

 and a cerebro-pedal connective and to a number of nerves. The 

 nerves that supply the anterior part of the mantle and the ante- 

 rior adductor muscle are especially easily seen. 



