416 THE MUSSEL CHAP. 



1. Cut away the left labial palps very carefully and look for the left 

 cerebro-pleural ganglion, situated just below the attachment of the 

 mantle-lobe close to the mouth and directly beneath the epithelium : 

 it may already have been exposed by removing the mantle-lobe. The 

 two cerebro-pleural ganglia are connected by a commissure above the 

 gullet. 



2. On the under surface of the posterior adductor muscle are the two 

 closely applied visceral ganglia, which will be at once seen when the 

 membranous covering of this muscle is dissected off. 



3. The pedal ganglia are more difficult to find, as they are deeply 

 situated, at the junction of the foot and visceral mass. Slit up the foot 

 by a median longitudinal incision, and carefully dissect the two halves 

 apart until the two closely apposed ganglia are seen ; then remove the 

 left half of the foot. 



4. Carefully dissect out the cerebro-pedal connective, which runs 

 straight from the cerebro-pleural to the pedal ganglion. Then follow 

 out the cerebro-visceral connective from behind forwards : each extends 

 forwards from the corresponding visceral ganglion along the inner 

 surface of the renal organ, and then passes slightly downwards across 

 the visceral mass to the cerebro-pleural ganglia. Sketch. 



VI. The sensory organs are not easy to make out. In order to see 

 the osphradium and tactile-cells, it is necessary to cut microscopic 

 sections. The statocyst may be found by examining under the microscope 

 a portion of the tissue just behind the pedal ganglion ; but it is much 

 more easily observed in the small fresh-water bivalve Cyclas, often to 

 be found in ponds : it is only necessary to examine the entire animal 

 in water under a low power after removing the shell in order to see 

 the two globular statocysts, each lined by epithelial cells and containing 

 a lithite, which is in constant, trembling motion. 



VII. The alimentary organs are imbedded in solid tissue in the visceral 

 mass, the hinder part of the intestine or rectum alone being surrounded 

 by a ccelomic cavity (pericardium). The anus has already been seen. 



i. Remove the animal entirely from the shell, and pin down 

 through the adductor muscles and foot under water. Dissect away the 

 epiderm to the left side of and above the stomach (if this has not already 

 been done), and note the brown digestive gland in the anterior dorsal 

 part of the visceral mass. Then insert a seeker into the stomach 

 through the mouth, to act as a guide, and slit up the short gullet and 

 stomach. Note the ducts of the digestive gland opening into the 

 stomach. 



