THE HARD-SHELL CLAM 87 



periostracum, on the outside. The two latter layers are secreted 

 by the edge of the mantle ; the periostracum is very thin and is 

 frequently wanting on some portions of the shell. Place a piece 

 of the shell in a solution of hydrochloric acid ; note the efferves- 

 cence which results. 



Exercise 4. Draw a view of the broken edge of the shell on a scale 

 of 5. Show the prisms of the prismatic layer. 



Place the animal in water and study it as it lies in the right 

 shell. ^ The two halves of the mantle will be seen to envelop en- 

 tirely the visceral mass of the foot. Over the dorsal portion of the 

 visceral mass the mantle is fused with it and cannot be separated, 

 but the lateral and the ventral portions of the mantle lobes hang 

 free, inclosing an extensive space, which is called the mantle cav- 

 ity. In this cavity, on each side of the visceral mass, lie the two 

 leaflike gills. Observe the edges of the mantle. They are fused 

 forward of the anterior adductor muscle ; the entire ventral edges 

 are free and permit the foot to protrude between them ; their pos- 

 terior edges are richly pigmented, and are also fused and modified 

 to form the two siphons. These are tubes which can be extended 

 beyond the shells and 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 is 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. 



1 For the study of the soft parts of the clam it is well to have also at hand a 

 specimen which has been deprived of both valves of the shell. 



