344 THE MAINTENANCE OF THE INDIVIDUAL 



innervates the dorsal muscles of the arms. As might be expected, 

 the tube-feet in the starfishes (Asteroidea) are supplied with sensory 

 organs. It is also interesting to note that at the tip of each arm of a 

 starfish there occurs a light-perceiving organ. 



Co-ordination by a Linear Nervous System 



Once the flatworms are reached in the evolutionary series one finds 

 the beginning of a linear type of nervous system. In the segmented 

 worms, or annelids, the nervous system is composed of two main 

 longitudinal, closely associated nerve trunks from which the several 

 branches in each somite pass laterally. Each segment of the worm 

 usually contains one, or, if the longitudinal cords are widely separated, 

 two ganglia arranged in parallel lines. In such cases the ganglia are 

 connected by a transverse commissure. This ladderlike type of nerve 

 co-ordination reaches its peak in the arthropods, where well-developed 

 ganglia occur in most somites. In nearly all types of the higher 

 invertebrates there is in the head end a ganglionic mass of nervous 

 tissue which has been dignified by the appellation of "a brain," 

 whereas it should have been more properly known, because of its 

 position, as a supraesophageal ganglion. All nerve cords of similar 

 type are ventral in position and lie beneath the gut. In order to reach 

 the supraesophageal ganglion, the nerve cord splits at the large 

 infra- or subesophagcal ganglion, and passes around the esophagus by 

 means of the circumesophageal connectives or loop. 



Reaching the arthropods, the primary change in the central nervous 

 system is found to be a greater concentration of ganglia. In the 

 larval forms of insects, there is little change from the linear nervous 

 system of annelids. In adult insects, however, ganglia are concen- 

 trated, and even fused, in the regions of special organs. For instance, 

 the "brain" and subesophagcal ganglia are connected with the ocelli, 

 antennae, and mouth parts, while thoracic ganglia are associated with 

 the wings and other appendages. An autonomic (sympathetic) nerv- 

 ous system, which is believed to control the action of the heart, 

 digestive system, and spiracle muscles, makes its debut in the 

 arthropods. 



Co-ordination by a Dorsal Tubular Nervous System 



Among the vertebrates there is a highly developed dorsal, tubular, 

 central nervous system with evidence, even in the lower forms, of 



