254 



COMPARATIVE ANATOMY. 



as also in many lower Crustacea, e. g. the Cirripedia (Fig. 129, B c); 

 in others, again, it is so much shortened that the cerebral and ventral 

 o-anglia form a single nervous mass, pierced by the oesophagus 

 (e. g. in Corycieidas). 



The ganglia of the ventral chain seem to be most regularly 

 distributed to the separate metameres in the Phyllopoda, which, in 

 this respect, retain most completely the primitive relations of parts. 



The ventral chord is in them 

 composed of a large number 

 of pairs of ganglia (about 60 

 in Apus), which follow one 

 another, gradually losing their 

 transverse as well as the longi- 

 tudinal commissures ; in the 

 Daphnida the ganglia have 

 the same characters, although 

 they are less in number, in cor- 

 respondence with the smaller 

 number of the metameres. 



Among the Thoracostraca, 

 the ganglia of the ventral 

 chord also are, for the most 

 part, distinct, but in corre- 

 spondence with the concre- 

 scence of the anterior meta- 

 meres into a more or less 

 extended cephalo thorax, the 

 anterior ganglionic masses are 

 fused ; this is either more or is 

 less distinctly expressed. Thus 

 in the Stomapoda (Fig. 128), 

 the ganglia which innervate 

 the anterior buccal, as well 

 as the prehensile feet (p), 

 form a larger complex (g 1 ) ; a 

 series of independent ganglia, 

 extending as far as the caudal 

 segment, is connected with 

 this (g u g m g™) r In the Deca- 

 poda Macrura, likewise, con- 

 crescence of the G cephalo - 

 common : while the (5 smaller 



Fig. 129. A Nervous system of a Crab 

 (Carcinus maanas). gs Cerebral ganglia, 

 o Optic ; a Antennary nerve. (Esophageal 

 commissure. i Transverse connection of 

 the commissure, gi Fused ventral medulla 

 (after Milne-Edwards). B Nervous system 

 of a Cirripede (C o r o n u 1 a diadema). 

 gs c gi as in A. a Antennary nerves which 

 are distributed in the mantle. Between 

 them is the " optic ganglion," connected with 

 the cerebrum, m Nerve to the stomach, s 

 Visceral nerve, which unites with a second 

 visceral nerve from the oesophageal ring to 

 form a plexus s" (after Darwin). 



thoracic ganglia seems to be very 

 ganglia of the abdomen still correspond exactly to the metameres. 

 Further fusion is seen in the thoracic ganglia of some Macrura 

 (Palinurus) \ in Pagurus the ganglia of the abdomen are represented 

 by one only, in correlation with the shortening of this region. So 

 too in the Brachyura all the ganglia of the ventral chain are fused 

 into a single one (Fig. 129, A gi). 



Reductions of this kind obtain, also, in other divisions of the 



