COMPARATIVE ANATOMY OF ORGANS. 639 



tion of a cell being muscular, the other nervous in its func- 

 tions. 



A more definite nervous organization is the disconnected 

 bodies and rod-like nerve-cells, and other nervous bodies 

 found near the eye-spots, and the nerve-cells and fibres at 

 the base of the sea-anemone ; but, as has been stated, a gen- 

 uine nervous system for the first time appears in certain 

 naked-eyed jelly-fishes, in which it is circular, sharing the 

 radiated disposition of parts in these animals. The Echiu- 

 oderms have a well-developed nervous system, consisting of 

 a ring (without, however, definite ganglia, though masses of 

 ganglionic cells are situated in the larger nerves), surround- 

 ing the oesophagus, and sending a nerve into each arm ; or in 

 the Holothurians situated under the longitudinal muscles 

 radiating from that muscle closing the mouth. 



In all other invertebrate animals, from the worms and 

 mollusca to the crustaceans and insects, the nervous system 

 is fundamentally built upon the same plan. There is a pair 

 of ganglia above the oesophagus called the " brain ;" on the 

 under side is usually a second pair ; the four, with the nerves 

 or commissures connecting them, forming a ring. This ar- 

 rangement of ganglia, often called the " oesophageal ring," 

 constitutes, with the slender nerve-threads leading away from 

 them, the nervous system of the lower worms, in many of 

 which, however, as also in the Poh/zoa and Brachiopoda, 

 the subcesophageal ganglia are wanting. Now to the 

 oesophageal ring with its two pairs of ganglia add a third 

 pair of visceral ganglia, and we have the nervous system 

 of the clam and many mollusks. In the higher ringed 

 worms, the Annulata, and in the Crustacea and Insects, a 

 chain of ganglia, or brains, which is ventral, lying on the 

 floor of the coelum or body-cavity, completes the highest 

 form of nerve-centre found in the invertebrate animals, 

 unless we except the mass of ganglia, partly enclosed in an 

 imperfect cartilaginous capsule of the Cephalopods, which 

 hints at the brain and skull of Vertebrates. The nervous 

 cord of the Appendicularia, an Ascidian, is constructed on 

 the same plan as in the Annulata, but the mode of origin and 

 .apparently dorsal position of the nervous system of the 



