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THE POPULAR SCIENCE MONTHLY. 



vertebrate animals, to use the word ganglion, which means a knot of 

 nerves. Really, it signifies a little brain, so that an insect or a mol- 

 lusk may have several brains in different parts of its body. It should 

 be remembered, also, that in anatomy the forward or anterior end of 

 an oyster is the part containing the mouth, and that is up against the 

 hinge, while the posterior part happens in this case to be near the 

 opening, or, as the oystermen call it, the nib of the shell. At b, then, 

 we see the large brain of the oyster called the posterior ganglion. 

 We see, too, that it is surrounded by nerves running to other parts of 

 the structure. There are two curved branches, marked c c, which con- 

 nect this brain with two comb-like objects. These are the nerves 

 of the branchiae or gills. This brain, then, has direct control of the 

 mechanism and functions of respiration. But it is noticeable that it is 



Fig. 8. The Nervous System op an Otstee. 



also connected with the entire system of the two nerve-lines, d d, 

 which suggest the spinal cord of the vertebrates. And this double 

 nerve-line crosses the two ganglia or little brains, a a, which are con- 

 nected by the transverse nerve-branch e ; thus the mouth, whose place 

 is shown by the half-moon, is encircled by a nerve-ring, and this regu- 

 lates the functions of ingestion. In those mollusks which travel, as do 

 mussels and scallops, there is a ganglion or locomotive centre. Bear- 

 ing this in mind, and the fact that the oyster does not have this gan- 

 glion, because it does not need it, not being a traveler at all, let us 

 give the gist of Dr. Todd's remarks on the nervous system of the mol- 

 lusca in general : " It affords a beautiful example of the complete 

 analysis of the more complicated nervous system of the vertebrata. 

 Have we not here distinctly marked out the cerebrum (the. centre of 

 volition and sensation), the medulla oblongata (the respiratory cen- 

 tre), and the cerebellum (the locomotive centre), as they occur in the 



