538 Dynamic Theory. 



upon a dry surface it will remain motionless, but if placed on the 

 water it will slide about as in life for half an hour. This indicates that 

 the reflex machinery of this insect is differentiated to stimulus of a pe- 

 culiar kind. The legs of the Centipede are stimulated by a dry surface, 

 but this one requires a wet one. Other sorts of stimulation are possi- 

 ble often to the same organs, causing different sorts of reactions. Thus 

 the presence of air in the lungs and windpipe stimulate the act of res- 

 piration, but the presence of some extraneous object, as dust or hairs, 

 causes a tickling and coughing. So in the case of the headless Centi- 

 pede, if his breathing pores, which lie along the side, be irritated by 

 the vapor of ammonia, or something of that kind, the body will bend 

 away as if to avoid it. 



FIG. 261. Nervous systemof Craw fish (Astacusfluvia- 

 tilis.) (Huxley.) 



a. Antennary nerve. 



6. Antennulary nerve. 



c. Circum-esophageal nerve. 



es. Esophagus in cross section. 



o. Optic nerve. 



syn Stomato gastric nerve. 



sa. Sternal artery in cross section. 



v Vent. 



1. Supra esophageal ganglion. 



2.-Infra 



6. 5th Thoracic 



7. Last 



13. Last abdominal 



We see in the foregoing examples, the ac- 

 tion of external agencies, especially touch or 

 contact stimulations, but also light, upon or- 

 ganisms, with the result of putting them into 

 motion in various ways. There can be no 

 mistaking the entirely reflex nature of the ac- 

 tion of one of these animals with its head off, 

 meaning by the term reflex that the stim- 

 ulation, in the form of a motion of a body 

 outside of the animal, sets up in the animal a 

 polar current, which passes from the point of 

 contact to a ganglion, and from thence to the 

 muscles of the limbs, and causes them to con- 

 tract and so give motion to the limbs. In 

 other words, the headless organism is a ma- 

 chine in the same way that a water-wheel is, 

 ^although a much more complicated one. But 

 when the head is on, it is still not a whit less 

 a machine, only its complexity is greatly in- 

 creased. 



The vertebrate body is constructed on the general plan of a worm, 

 and, like it, is composed of a series of segments. These segments are 

 arranged along a longitudinal axis called the spinal column. This is 



FIG. 261. 



