V.] THE FRESH-WATER MUSSEL. 329 



lobe of an animal which has been killed under 

 chloroform. Conspicuous among the cells thus 

 isolated there will be found large oval or rounded 

 ones, each lodging a central refractive globule. 

 b. Remove the mantle-lobe from a mussel which has 

 been killed as above and subsequently preserved 

 in alcohol, and split it into two. Transfer a 

 portion of one half to a glass slide, torn surface 

 upwards, and treat with iodine solution; the 

 whole will be studded with small brown spots. 

 Examine under a high power ; the spots are seen 

 to be identical with the globules observed in 

 a. (their chemical reactions are those of gly- 

 cogen). 



I. The nervous system. 



1. Pin the animal down so as to get the body absolutely 

 rigid ; remove the mantle-lobe and gills of one side 

 and slit open the organ of Bojanus. 



Wash until quite clean and examine under water. 



a. Find the cerebro- splanchnic commissures; two parallel 

 white cords traversing the excretory organ. Trace 

 the near one backwards; it passes round the 

 posterior retractor tendon towards the under side 

 of the posterior adductor muscle. Turn this latter 

 over, so as the better to display its under surface, 

 and note 



b. the parieto-splanchnic ganglia; two elongated yellow 

 masses confluent in the middle line, seen on 

 removing the membranous investment from the 

 ventral surface of the above-named muscle. They 

 distribute branches to the same, to the gills and 

 pallial lobe. 



