350 COMPARATIVE ANATOMY CHAP. 



of 6 united ganglia, i.e. of the mandibles, 2 maxillae, and 3 maxillipedes. 

 The last swelling of this infra -oesophageal ganglionic mass (the 3d 

 maxillipedal) is pretty clearly marked off. The 5 distinctly separate 

 large thoracic ganglia for the 5 posterior thoracic segments and their 

 extremities (the ambulatory feet) follow, and of these the 4th and 

 5th ganglia are very near each other. In the abdomen we find 6 

 ganglia, the last is the largest, and, as in all Malacostraca, may well 

 represent 2 or even more originally separate ganglia. In the Cray-fish, 

 as in nearly all Thoracostraca, the 2 ganglia of the originally double 

 ganglion and the longitudinal commissures between the consecutive 

 ganglia of the ventral chord are so closely connected in the middle 

 line that their double nature is not outwardly perceptible. The 

 longitudinal commissures separate from each other only between the 

 penultimate and antepenultimate thoracic ganglia (6th and 7th), so as 

 to let the sternal artery pass between them. 



In a few Macrura the 2 most posterior thoracic ganglia are fused, or there is a 

 close approximation of all the cephalo-thoracic ganglia (Carididce, Palinurus). Here, 

 however, the 6 abdominal ganglia remain separate, while in the Paguridce only 1 

 abdominal ganglion is present, in correspondence with the great reduction of the 

 abdomen. We finally come to the Brachyura (Fig. 236, F), in which, in 

 correspondence with the great reduction of the abdomen and the concentration of the 

 cephalo-thorax, the whole ventral chord is fused together into a great thoracic knot, 

 from which numerous nerves radiate out to all sides. 



Sympathetic Nervous System. 



This seems to occur in all Malacostraca, at least it has been observed 

 with considerable uniformity in representatives of the three principal 

 divisions, the Leptostraca, Arthrostraca, and Thoracostraca. In the 

 Cray-fish a nerve with double roots arises on each side out of the 

 commissural ganglion, proceeds forwards to the upper lip and mounts 

 upwards at the sides of the oesophagus. The two nerves unite on the 

 upper side of the stomach to form a median nerve which swells into a 

 ganglion. From this ganglion a branching nerve runs backwards and 

 spreads out in the wall of the stomach and gives off branches to the 

 liver and also probably to the heart. The sympathetic nervous 

 system is further connected by an unpaired nerve with the posterior 

 portion of the brain. 



So highly developed a sympathetic nervous system seems wanting 

 in the Entomostraca. But it is noteworthy that in the Phyllopoda 

 (Branchipus) a nerve arises on each side out of the cesophageal com- 

 missures which runs to the upper lip. The 2 nerves unite to form a 

 labial ring, which is connected with a median ganglion and gives off 

 nerves to the upper lip, the muscles of the oesophagus, etc. 



Structure of the Brain. The brain of the Crustacea rises in the Malacostraca to a 

 very high degree of complication. This complication, which reaches its highest point 

 in the Decapoda, is seen in the complicated arrangement of the ganglionic cells and 



