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 Echin- 
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 esophagus, 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 cesophagus called the ‘‘ brain ;’? on the 
under side is usually asecond pair ; the four, with the nerves 
or commissures connecting them, forming a ring. This ar- 
rangement of ganglia, often called the ‘‘ esophageal 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 Polyzoa and Brachiopoda, 
the subcesophageal ganglia are wanting. Now to the 
esophageal 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 celum 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 
