CHORDATES IN GENERAL 147 



comes expanded and modified to form the brain. The continuous 

 tubular nerve cord is at the apex* of the development of centraliza- 

 tion in the nervous system, and allows for an increase in number 

 of nerve cells, increased accessibility, and more intimate association 

 of ganglionic masses to furnish better coordination. These are all 

 advances in both structure and function when compared with other 

 groups. The chordates possess segmentation (metamerism), but it 

 is progressively obscure as one proceeds from simpler to more com- 

 plex forms. There is a tendency toward fusion of metameres and 

 shifting of superficial muscles. The internal skeleton of this group 

 compared with the external one of others to be studied does not give 

 as great a leverage for the muscles, but it greatly increases the mechan- 

 ical freedom allowed and this is a distinct advantage as well as an 

 advance in structure. 



Classification 



There are approximately 40,000 different species in this phylum 

 which is divided into four established subphyla as follows : 



Hemichorda (hem i kor' da, half cord) or sometimes known as En- 

 teropneusta (en ter op nus' ta) includes order Balanoglossida with 

 its four families, ten genera and twenty-eight species, and order 

 Cephalodiscida with its two genera C ephalodiscus and Rhahdopleura. 

 These are all small wormlike animals. 



TJrochorda (u ro kor' da, tail cord), or Tunicata (tu ni ka' t^) 

 includes the tunicates, all of which are marine and mostly small. 

 Adults show a high degree of degeneration so it is the larvae only 

 that exhibit distinctive characteristics of the phylum. Molgida, 

 Cynthia, Appendicularia, and Salpa are examples. 



Cephalochorda (sef a 16 kor' da, head cord) includes approximately 

 twenty-eight different species of marine, shore-loving, fishlike forms 

 of which Amphioxus (Branchiostonia lanceolatus) is the most common 

 representative. 



Vertebrata (ver te bra' ta, jointed) animals with backbone — frog, 

 man. These are the larger, more conspicuous animals. In most recent 

 classifications this subphylum is divided into seven classes ; however, 

 the second is sometimes found as a subclass under the third. These 

 classes are as follows: 



Cyclostomata (si klo sto' ma ta, circle and mouth). Round-mouthed 

 fish with only median fins, unsegmented notochord, and jawless. 

 Lampreys and Hagfish. 



