308 AMETZOAN PHYLA 



pharyngeal slits offer a mode of respiration more effective than that 

 allowed by any type of respiratory organ possessed by lower forms 

 because the slits are interposed directly in the path of the circulation 

 and all of the blood in the body passes through them. In tracing the 

 development of the earlier phyla it has been evident that their nervous 

 systems have advanced in the degree to which they have been centralized, 

 thus bringing about more effective coordination. The development of 

 a continuous tubular nervous system makes possible the most intimate 

 association of ganglionic masses and thus the greatest degree of interaction 

 between them. A continuous tubular nervous system also offers more 

 surface for the escape of fibers than do separate metamerically arranged 

 ganglia, thus permitting an increase in the number of nerve cells. The 

 obscuring of the metamerism is in the interest of unified action of 

 the body. The arthropods develop the possibilities of metamerism to the 

 maximum degree. The chordates retain the advantages of the metameric 

 plan but relinquish it to the degree desirable to secure the greatest 

 unity in the operation of the body as a whole. Other advances possessed 

 by the vertebrates but not shared by the lower chordates remain to be 

 discussed later. 



336. Classification. — The chordates are usually separated into 

 four subphyla, as follows: 



1. Hemichordata (hem i kor da' ta; G., hemi, half, and chorde, cord), 

 or Adelochorda (ad e lo kor' da; G., adelos, concealed, and chorde, cord). 

 Includes two or three types of marine wormlike animals, which are 

 all small, some very minute. 



2. Urochordata (u ro kor da' ta; G., oura, tail, and chorde, cord). 

 Includes the tunicates, also marine, which illustrate extreme degener- 

 ation and, in a sense, retrogression. 



3. Cephalochordata (sef a 16 kor da' ta; G., keyhole, head, and chorde, 

 cord). — Includes a marine type known as the amphioxus which has a 

 somewhat fishlike form. 



4. Vertehrata (ver te bra' ta; L., vertebra, a joint). — Includes fish, 

 amphibia, reptiles, birds, and mammals. 



