92 INVERTEBRATE ZOOLOGY 
lie the two leaf-like gills. In front of the gills are two pairs of 
triangular flaps, the oral palps, between which, in the median 
line and just back of the anterior adductor muscle, lies the 
mouth. Find it. 
Trace the irregular line of attachment of the mantle with the 
visceral mass ; it follows the base of the gills and of the oral palps 
and passes beneath both adductor muscles. Observe the edges 
of the mantle and note that at the hinder end of the animal they 
are darkly pigmented, and the middle point of the pigmented 
line is joined with the base of the gills by a short septum. This 
septum divides the posterior portion of the mantle cavity into 
a dorsal and a ventral chamber. The latter is the very large 
branchial chamber which contains the gills; the former is the very 
small cloacal chamber. The pigmented edges of the mantle are 
at this place modified to form, when the edges of the two sides 
of the mantle are applied to each other, two short tubular 
openings, which place these two chambers in communication 
with the outside water and are called the siphons. The ven- 
tral siphon is called the branchial or incurrent siphon; through it 
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 charged with 
feecal matter from the alimentary tract and carbon dioxide of 
respiration. Note the sense tentacles on the branchial siphon. 
Probe the dorsal siphon. Carefully remove the left lobe of 
the mantle after cutting it with fine scissors along its line of 
attachment with the visceral mass. 
Through the transparent body-wall observe the organs in the 
dorsal portion of the visceral mass. Just back of the anterior 
adductor muscle is the liver, which can often be recognized by 
its greenish color, and back of which is the dark-colored Keber’s 
organ. Between the hinge ligament and the base of the gills lies 
the heart in its transparent pericardium, and beneath it is the dark- 
colored kidney. The rectum may be seen passing through the 
