GENERAL ORGANOLOG Y. 



147 



granular substance." From the central nervous mass run 

 out the peripheral nerves, and also the commissures, the 

 cords connecting similar ganglionic masses. 



sir 



FIG. 71. A pair of ganglia from the ventral nerve-cord of Phronima sedeniaria. In the 

 centre is Leydig's granular substance with a superficial layer of ganglion-cells (MGl 

 VGl, HGl, Rz) ; ZA~, longitudinal commissure ; SN, efferent nerve-fibres . (After Claus. 



Supracesophageal (or Brain) Ganglia. Since now 

 most animals are symmetrically built, the ganglia are found 

 grouped in pairs ; a left and a right ganglion correspond to 

 one another and are connected simply by a cord of nerve- 

 fibres, the transverse commissure. Of most constant 

 occurrence are two ganglia, which lie dorsally over the 

 pharynx, and hence are called the snpra-cesopliageal ganglia 

 or brain ganglia. If still other ganglia occur, they lie 

 ventrally and below the digestive tract (ventral nerve-cord). 



Ladder Nervous System. A widely-recurring ar- 

 rangement is that termed the ladder nervous system (of 

 annelids and arthropods) (Fig. 72). Numerous pairs of 

 ganglia (in the example before us, nine) lie in serial 

 arrangement on the ventral side of the animal, and are con- 

 nected by longitudinal commissures, and also by left and 

 right commissures connecting the left and right ganglia. 

 The first pair of the series is formed by the subcesophageal 

 ganglion, which sends out commissures right and left, sur- 

 rounding the pharynx, to the supra-oesophageal ganglion. 

 The supra- and subcesophageal ganglia together with the 



