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 

 faecal 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 



