. Elements of Comparative Anatomy. 256 COMPAEATIVE ANATOMY. Among the Araclinida the ventral ganglia are very often reduced and fused. They are all characterised by the close connection between the cerebral ganglia, and the venti'al chord, owing to the extreme shortness of the commissures. The nervous system is most richly segmented in the Scorpions. The feebly developed cephalic ganglion gives off two short commissures to the ventral chain, which consists of 8 ganglia. The first of these is remarkable for its size, and appears to be homologous with the single large ganglion in the cepha- loth
. Elements of Comparative Anatomy. 256 COMPAEATIVE ANATOMY. Among the Araclinida the ventral ganglia are very often reduced and fused. They are all characterised by the close connection between the cerebral ganglia, and the venti'al chord, owing to the extreme shortness of the commissures. The nervous system is most richly segmented in the Scorpions. The feebly developed cephalic ganglion gives off two short commissures to the ventral chain, which consists of 8 ganglia. The first of these is remarkable for its size, and appears to be homologous with the single large ganglion in the cepha- lothorax of the Spiders. As in them it is the point of origin of the pedal nerves, and must therefore be considered as com- posed of several ganglia. The three suc- ceeding ganglia are also placed in the cephalothorax, and the four last, which are widely separated from one another, are found in the segments of the tail. In the Galeodea and Phrynida, as in the Aranea, the ganglionic chain is re- placed by a large ventral ganglion, which (Fig. 130, i) is, especially in the Spiders, of a radiate form, and gives off the nerves for the ventral appendages, and also two which run into the abdomen; in the Galeodea branched nerve-trunks are sent to the segments of the abdomen. In all these divisions the cerebral gang- lion, which is generally distinctly paired, and in the Galeodea (,s) especially large, gives off nerves for the eyes; close by the optic nerves those for the chelicerae arise in the Spider, and thus we see that these organs are metamorphosed antennae. A complete concentration of the central portions of the nervous system is seen in the Acarina, where the cerebral ganglia, which are often feebly developed, may be replaced by a commissure. The ventral medulla, which is large and forms a single mass, exhibits numerous traces of segmentation in the arrangement of the gang- lionic cells and fibrous elements ; it gives off nerves all round. The simple character of the nervous
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Keywords: ., bo, bookcentury1800, bookdecade1870, bookpublisherlondonmacmillan
