96 IX VERTEBRATE ZOOLOGY 



muscle ; the entire ventral edges are free and permit the foot to 

 protrude between them ; their posterior edges are richly pig- 

 mented, and are also fused and modified to form the two 

 siphons. These are protrusile tubes, through which water is 

 taken into and expelled from the mantle cavity. Probe them. 

 Note on each side below the posterior adductor muscle the 

 triangular muscle which connects the siphons with the shell. 

 It is the siphonal retractor muscle. Between the two siphons in the 

 mantle cavity note the short transverse septum which divides the 

 posterior portion of the mantle cavity into two chambers, a dorsal 

 and a ventral one. The latter is the very large branchial chamber, 

 which contains the visceral mass and the gills, the former, 

 the very small cloacal chamber. The ventral siphon is called the 

 branchial or incurrent siphon; througli it the water streams into 

 the branchial chamber bearing food and air for respiration. The 

 dorsal siphon is called the excurrent or cloacal siphon and through 

 it water passes outward from the cloacal chamber charged with 

 carbon dioxide of respiration and with faecal matter from the 

 alimentary tract. Probe the cloacal chamber. 



Carefully remove the left mantle lobe after cutting it with 

 fine scissors at its line of attachment, beginning at the forward 

 end. Cut off the siphonal muscle, leaving the siphon in posi- 

 tion. Place the animal in water and study the arrangement of 

 the organs. Observe the position of the gills; note in front of 

 them two triangular flaps, the oral palps; in the median line 

 between the two pairs of oral palps is the mouth; find it. 

 Along the base of the gills note an elongated passage leading 

 posteriorly to the cloacal chamber, the suprabranchial passage of 

 the outer gill. Blow into this passage at its hinder end in the 

 cloacal chamber with a blow-pipe, or probe it. 



Observe again the siphonal region. Note the short septum 

 which separates the branchial from the cloacal chamber, and the 

 opening between it and the visceral mass ; probe this opening. 

 Just beneath the umbo will be seen through the semi-transparent 



