Nervous System. 535 



Tape worms, Liver Flukes, Planarians, Ribbon worms, Hair worms, 

 Nematodes, &c. , are examples of this stage of development. Among 

 those worms which consist of a series of segments joined together one 



FIG. 256.^ Turhellarian 

 worm, Me&ostomum Ehreriber- 

 gii, 



g Paired Ganglia connected 

 by commissure. 



e. Eye spots or pigments. 



t. Nerve trunk. 



m. Mouth with pharynx. 



a. Alimentary canal. 



s. Skin covered with cilia. 



FIG. 257. Jlrain and anterior 

 part of the ganylionic chain of 

 a Tube-toormSerpula. 



G. Cerebral ganglion (brain). 



C. Esophageal ring. 



7#.--Sub-esophageal ganglion. 



e.- Nerves to the tentacles and 

 mouth segment. 



a. Nerve chains with gang- 

 lions at each segment of the 

 body. 



behind the other, and 

 which are classified under 

 the sub-kingdom Annu- 

 losa, we find a pair of Fia. 257. 



nervous ganglia accompanying each one of the 

 segments, and another encircling the neck and 

 gullet, the whole joined to each other by fibres on 

 each flank of the body, running from one end to 

 the other. Thus, as t"he body is increased in the 

 number of its parts, a harmony of movement and 

 co-operation of action is secured among them by the balancing of the 

 stimuli and their mutual reinforcement or partial elimination in the 

 nervous ganglia. 



FIG. 268. Sea-Squirt (Ascidian). 



a. Mouth. 6. Vent. c. Nervous system. 



d. Muscular sac containing stomach, &c. 



The ganglion of the Ascidian, fig. 258, when stimu- 

 lated causes the contraction of the muscular bag which 

 envelops its -viscera and suddenly empties it, causing the 

 '< squirt," which gives the animal its common name. In 

 Mollusks, generall}', there are two masses of ganglionic 

 matter, one above and the other below the esophagus. 

 These are connected with each other to form a ' < throat 

 ring," and from them nerve fibres radiate to the muscles, 

 and such sense organs as they possess. Shell-fish like 

 the pond Muscles, have n large ganglion and a sense organ in the fleshy 

 projection called the foot, which is connected to the cerebral ganglion. 

 The cephalopods (Cuttle fishes, &c. ) have a central ganglion which 

 answers to the brain, and possess also an independent ganglion in each 



FIG. 256. 



FIG. 268. 



