

A Lamellibranch. 55 



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The Muscular System. The strong anterior and pos- 

 terior adductor muscles have already been noted. Is there 

 any difference in their relative size ? Can you discover the 

 retractors of the foot, smaller muscles lying near the adduc- 

 tors? The siphonal muscles lie around the base of the 

 siphon. Note any lines of attachment on the inner surface 

 of the removed valve that correspond to the muscles just 

 mentioned. 



The Respiratory System. Pass a probe into the 

 lower siphonal canal (the incurrent canal} and note that 

 the canal may convey water into the mantle chamber. Into 

 this chamber, which is also known as the infra-branchial 

 chamber, the gills or branchiie hang. Each gill is made up 

 of two parallel plates of rods or bars. Study carefully the 

 attachment of the two pairs of plates of each side to the 

 mantle, without, and to the body- wall, within. 



Inject water into the upper siphonal opening and notice 

 that it fills a certain chamber lying above each pair of gill- 

 plates or lamellae. This chamber is the supra-branchial 

 chamber, and on each side, above the outer and inner gills, 

 has an outer and an inner division. The outer subdivision 

 may be exposed by cutting along the line of attachment of 

 the outer lamella to the mantle. The inner sub-chamber 

 may be exposed by turning back both gills and cutting along 

 the line of attachment of the inner lamella of the inner gill 

 with the body-wall. 1 Look into these chambers and note 

 the numerous openings of the water-pores, tubes which lead 

 from between the gill-lamellae. 



Carefully slit open a gill in such a way as to separate its 

 lamellae and, floating one of the lamellae on a slide, exam- 

 ine with a compound microscope. If the gill is still alive, a 



1 The inner lamella is only partially united to the body-wall. 



