ANIMAL KINGDOM. 247 



from the surface of the body, will produce conclusions still 

 more vague and unsatisfactory. 



The nervous system of the Mollusca is very peculiar, 

 consisting of a certain number of medullary masses di- 

 spersed in different parts of the body, and of which the 

 principal one, commonly called the brain, is situated over 

 the oesophagus, which it encompasses with a nervous 

 collar. The most general notion that we can form at pre- 

 sent of the nervous system of the Mollusca is, that the 

 medullary collar must always in its circumference contain 

 four ganglions, which may either be united two and two, 

 as it is probable they are in the Acephala, or all four to- 

 gether, as they are in the genus Tritonia. The brain is 

 always composed of two of these lobes, which are generally 

 connected, as in Aplasia, though sometimes separate, as 

 in Haliotis and Patella. The remaining two ganglions 

 of the collar send off nerves to the organs of respiration, 

 &c. and are either united together, as in the common slug 

 and Patella, or are separate, as in Aplasia. In the genus 

 Tethys one lobe of the brain seems to be joined to one lobe 

 of the inferior ganglions, that is, two and two together. 

 There are often other ganglions distinct, such as two for 

 the mantle, which appear however more particularly to 

 belong to the Cephalopoda, and consequently are most ma- 

 nifest in those Mollusca which are the nearest to them in 

 natural affinity. But whether these ganglions are united 

 in the other Mollusca to the two inferior ganglions before 

 mentioned, and so may have escaped detection, or whether 

 they are altogether annihilated in these more imperfect 

 animals, it must be left for future anatomists to decide. 

 We may however be permitted to observe on the whole, 

 that the nervous system of the Mollusca cannot at present 



