50 THE STRUCTURE OF 



known as ' commissures.' Such transverse commissures 

 always unite similar ganglia, whether these are so close 

 as to be more or less continuous, or distinctly separated 

 from one another. Two or three illustrations will suffice 

 to make these bilateral arrangements more intelligible to 

 the reader. 



In some of the Nudibranchiate Mollusks so common on 

 the sea- shore, there is in front and on each side a large 

 roundish though functionally compound ganglion receiving 

 numerous ingoing nerves and connected with its fellow 

 by means of a very thick and a thin commissure (fig. 17). 

 The sensory ganglion is also connected on each side by 

 means of a short commissure with its own motor ganglion, 

 from which outgoing nerves proceed to the muscles, and 

 the two motor ganglia are in their turn connected by a 

 longer transverse commissure (fig. 17, e). 



Thus, in each half of the body of one of these ani- 

 mals there is a complex aggregate of the mechanisms for 

 reflex actions represented by ingoing fibres entering a 

 sensory ganglion in connection with a motor ganglion, 

 together with outgoing fibres issuing from the latter. 

 Whilst in addition, the two halves of the nervous system 

 are united to one another by the above-mentioned trans- 

 verse commissures. It is by virtue of these connections 

 between the respective ganglia of the two sides that a 

 properly co-ordinated activity of the whole body is rendered 

 possible, in response to sensory stimuli. 



In other animals, such as the Grasshopper, whilst the 

 bilateral symmetry of the nervous system (fig. 18) is just 

 as obvious, it is much more complex and more developed 

 longitudinally. The sensory and motor ganglia are nume- 

 rous and are arranged side by side in serial order, though 

 many of them are more completely fused with one another 

 and with those of the opposite side than is the case with 



