416 THE MUSSEL 



CHAP. 



versely through the gill-filaments. Compare with Fig. 102, 

 and sketch. 



3. Mount in salt-solution a small piece of the edge of a 

 fresh gill, and also of the mantle, and observe the move- 

 ments of the cilia. 



IV. If your specimen is a female, and contains ova or 

 larvae in the outer gill-lamina, examine some under the 

 microscope. The ova are provided with a canal or micropyle 

 perforating the egg-membrane, for the entrance of the 

 sperms. Note the form of the larvae or glochidia (Fig. 105). 

 Sketch. 



V. The nervous system consists of three pairs of small 

 orange-coloured ganglia, with connectives between them. 



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-apposed 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-pcdal 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 



