106 INVERTEBRATE 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; through 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 fecal 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 
