COMMON CRAYFISH. 189 



The nerve-factors which make up the stomatogastric system are derived from 

 two sources, from the supra-oesophageal ganglion itself, and from the commissures 

 connecting it to the infra-oesophageal ganglion at the spot where these commissures 

 come into contact with the walls of the oesophagus. The nerves derived from the 

 first-named source are two, a superior azygos nerve and an inferior azygos nerve. 

 The former has a small ganglion close to its origin ; it is short and runs upwards, 

 i.e. dorsally. The latter is long and runs backwards and downwards. Two small 

 ganglia, the mandibular or oesophageal ganglia, recently investigated by Krieger, give 

 origin to the second set of factors named above. They are semi-oval and lie on the 

 ventral side of the commissures, and from each of them spring at least three nerves. 

 One is external and bends down, branching on the oesophagus ; the two others, the 

 superior and inferior roots, are internal and pass forwards between the commissures. 

 The inferior roots unite together to form a single trunk, to which the superior roots 

 then unite. The single nerve thus formed is joined by the inferior azygos nerve from 

 the supra-oesophageal ganglion, and constitutes a median nerve which runs upwards 

 in front of the stomach, giving off one after another three branches to that organ 

 (Lemoine). It then unites with the superior azygos nerve l from the supra- 

 oesophageal ganglion. The single trunk formed by this union bends round the 

 stomach on to its dorsal aspect. Close to the point of union it gives a nerve to the 

 stomach and twigs to the anterior gastric muscles through which it passes. It then 

 forms the stomatogastric ganglion, from which spring two nerves, an upper, the car- 

 diac nerve of Lemoine, lodged in the integument and going to the heart, and an in- 

 ferior or gastro-ttepatic' which lies on the dorsal wall of the stomach. The latter 

 passes backwards and divides posteriorly into the terminal branches. Between 

 these and the stomatogastric ganglion is a slight swelling from which rise the two 

 lateral branches. Both lateral and terminal branches pass downwards. The latter 

 supply the posterior gastric muscles. They eventually distribute themselves to the 

 liver so-called and various stomachal muscles, and anastomose both with the lateral 

 branches and \hepostero-lateral nerve which arises from each oesophageal commissure 

 dorsally to the oesophageal ganglion, and passes upwards on the oesophagus. These 

 various nerves give off numerous branches which have recently been investigated in 

 detail by Mocquard in various Decapoda. 



The ganglia consist of central masses of Leydig's ' punkt-substanz ' formed by 

 dense networks of fine nerve-fibrils, and external masses of ganglion cells, varying in 

 size. The ganglion cells themselves differ in the same respect : the smallest possess 

 but little protoplasm. Each cell is contained in a connective tissue capsule. Their 

 processes, though numerous, originate in most instances from one surface or pole. 

 The nerves are tubular, and, according to Krieger, consist of an external sheath and 

 homogeneous fluid contents, but Freud states that there are delicate fibrillae im- 

 bedded in this homogeneous substance ; and he traces the same distinction, viz. a 

 homogeneous matrix and imbedded fibrillar network, in the bodies of the ganglion 

 cells. The nerves branch repeatedly. There is a tough elastic perineurium or 

 common investment, composed of decussating fibres and covered within and without 

 by cellular connective tissue. 



Histology of nerve-cord, nerves, &c. In Astacus, Krieger, Z. W. Z. xxxiii. 1880 ; 



1 This root is not mentioned by Mocquard, but is figured by both Krieger and Lemoine, and I 

 have found it myself more than once. 



