NERVOUS SYSTEM. 189 



these animals, notwithstanding their molluscous exterior, 

 fixed in a multivalve shell lined by a fleshy mantle, are con- 

 nected with the articulated classes by the diplo-neurose cha- 

 racter of their nervous system, and by all their other organs 

 of relation. 



The annelida present as great diversities in the extent of 

 development of their nervous system, as in their exterior 

 forms and their whole internal organization. In the minute 

 transparent bodies of the planaria, and in many of the 

 simplest forms of aquatic worms, scarcely a trace of nervous 

 columns can be perceived, although their organs of vision are 

 often numerous and obvious, and even in the naids and some 

 of the nereids the nervous chord along the middle of the ven- 

 tral surface of the trunk appears as uniform and simple, as in 

 the nematoid entozoa. The columns in the simpler anne- 

 lides, as in the first larva state of the higher entomoid 

 classes, scarcely present ganglionic enlargements in their 

 course along the abdomen, or supra-cesophageal ganglia at 

 their anterior extremity, from the general inferiority of their 

 organization, and from the still imperfect development of 

 their lateral appendices for progressive motion, and of their 

 cephalic appendices for sensation or mastication. In the 

 long cylindrical and muscular bodies of the air-breathing 

 earth-worms (Fig. 82. D,) with their myriad of short and 

 highly moveable segments, their exquisitely sensitive skin, 

 and their minute rudimentary feet, we can perceive innumer- 

 able mixed nerves (Fig. 82. D. b, ,) proceeding laterally 

 from the closely approximated nervous columns, but scarcely 

 a ganglionic enlargement is developed in their long and 

 equal course along the ventral surface of the trunk. In this 

 highest of the helminthoid classes the nervous columns are 

 still entirely enclosed in the cyclo-vertebral elements along 

 with all the other viscera of the abdominal cavity. The 

 abdominal chords of the earth-worm embrace the oesophagus 

 at their anterior extremity, and form two distinct cephalic or 

 supra-aesophageal ganglia (Fig. 82. D. a,) which supply 

 nerves to the large muscular apparatus of the mouth, and to 

 the long dorsal sympathetic. These two cephalic ganglia lie 

 in contact with each other, are lengthened transversely, like 

 those of insects, appear grey-coloured on the surface, and more 

 white internally, and seem to be composed of minute globules 



