NERVOUS SYSTEMS OF INVERTEBRATES 



-V.A. 



V.G< 



some authorities hold a different view as to the antennules, be- 

 lieving that these have always been situated in front of the mouth, 

 and are in reality outgrowths from the head-lobe. If so they are 

 comparable to the sensitive palps on the head of a Nereis, and 

 the brain of Apus is strictly equivalent to the brain of an Annelid. 

 We shall assume here the truth of the first 

 view, as the balance of evidence is in its favour. 

 The body of Apus is made up of a com- 

 paratively large number of segments, while in 

 Crayfish and Crab, as in all members of the 

 highly -organized group (Decapoda) to which 

 they belong, there are relatively few, i.e. 

 twenty, so far as can be definitely made out. 

 Five belong to the Head, eight to the Thorax, 

 and seven to the Abdomen, each with a pair 

 of ganglia and, except the last, provided with 

 a pair of limbs. The nervous system of the 

 Crayfish has undergone a certain amount of 

 fusion and centralization (fig. 1015). The brain 

 is larger and more complex than that of Apus, 

 and it supplies not only the first but also the 

 second feelers, the ganglia corresponding to 

 which have shifted along the nerve-ring. Even 

 greater fusion has taken place at the front end 

 of the ventral cord, where there is a large 

 ventral ganglion, which has resulted from the 



r i 1 i . ril 1 Fig- 1015. Central Nervous 



union of the last three pairs 01 head-ganglia system of crayfish 

 (supplying the three pairs of jaws), and the G-> Guiiet (in cross sec- 



^% & . r1 . t- / I tion); Br., brain; N.R., side 



three first pairs of thoracic ganglia (supplying O f nerve-ring; V.G., ventral 



11 . r r \ T ganglion; iii-vin, ganglia of 



the three pairs of foot-jaws). It is interesting third to eighth thoracic seg . 



h-3 



+ 7 



V.P- 



vp - f nterior and p s ' erior 



visceral nerves. 



to notice that the third thoracic ganglia are 

 caught as it were in the act of uniting with 



f 



those in front of them. The last five pairs of 

 thoracic ganglia (supplying pincers and walking-legs) are clearly 

 defined, although by reference to the figure it will be seen that 

 the last two are beginning to unite, while just in front of this 

 the doubleness of the cord is practically demonstrated by the 

 fact that its two halves diverge, for the passage of an artery 

 which runs vertically downwards from the heart to supply the 

 ventral region of the body. The first five pairs of abdominal 



