VII PRACTICAL DIRECTIONS 401 



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

 mantle lobe close to the mouth and directly beneath the eijithelium : — 

 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 more 

 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 osphraditim and tactile-cells, it is necessary to cut microscopic 

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

 a portion of the tissue just behind the pedal ganglion ; but is 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 otocysts, each lined by epithelial cells and containing 

 an otolith, which is in constant, trembling motion. 



VII. The alimentary organs consist of a gullet, stomach, paired 

 digestive-gland, and coiled intestine, imbedded in solid tissue in the 

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

 surrounded by a coelomic cavity (pericardium). The anus has already 

 been seen. 



I. 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. 



Pract.'Zool. D -D 



