LESSON 71.] NERVOUS SYSTEM. 237 



1075. In the Ascarides, a dorsal nervous chord is continued from 

 the oesophageal ring, down the middle line of that aspect of the body 

 corresponding to the ventral chord on the opposite aspect. 



1076. The nervous system of the ANNELLIDA, or red-blooded 

 worms, as they are commonly called, presents a marked advance be- 

 yond its condition in the white-blooded parasitic worms ; it consists 

 of a double median central chord, or chain of small ganglions, ex- 

 tending from one end of the body to the other ; the two chords di- 

 verge anteriorly to allow the passage of the oasophagus, and again 

 unite above that tube to form a distinct, though small, bilobed ce- 

 phalic ganglion. 



1077. Most of the Annellides are provided with ocelli (eyes), and 

 in many of them the head supports soft cylindrical tentacles (feel- 

 ers), called (improperly) antennw : they are pj& g62 

 obviously organs of touch, but differ from the 



antennae of insects in the absence of joints. 



1078. In this class the nervous system has 

 reached a higher type and more constant plan 

 of arrangement. 



1079. It always commences by a symmet- 

 rical bilobed ganglion, which, both by its situ- 

 ation above the mouth and the parts which it 

 supplies, merits the name of brain, which it 

 has generally received. 



1080. In the medicinal leech there are sent 

 off from this ganglionic centre (Fig. 362, a), 

 ten distinct optic nerves (6, 5), Besides many 

 smaller filaments to the integument and other 

 parts of the head ; each optic nerve or fila- 

 ment terminates by expanding upon the base 

 of a black eye-speck or ocellus, ten of which 

 may be distinguished by the aid of a lens of 

 moderate magnifying power, dotting, as it were, 

 at equal distances, the upper margin of the 

 expanded suctorial lip. 



1081. The principal nerves which arise 

 from the brain of the leech are what may be 

 called analogically the crura, which diverge as 

 they descend to embrace the oasophagus, and 

 are often called the oesophageal chords ; they 



. , , . . . . , , Nervous system of the 



then converge and reunite to join the large medicinal Leech. 



