PHYLUM CHORDATA 361 



cord and brain. In higher forms the anterior end of the tube be- 

 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 studied does not give as 

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

 cal 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 (hemikor'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 Cephalo- 

 discida with its two genera C ephalodiscus and Bhahdopleura. These 

 are all small wormlike animals. 



Urochorda (u r6 kor'da, tail cord), or Tunicata (tunika'ta) in- 

 cludes 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. There are 

 three classes: (1) Larvacea, so named because it retains the larva 

 form throughout life. Genus Appendicidaria is an example. (2) 

 Ascidiacea, the sea squirt, either free-swimming or sessile, simple or 

 colonial, may reproduce sexually or by budding. Molgula, Cynthia, 

 and Ascidia are common examples. (3) Thaliacea, free-swimming, 

 pelagic, solitary or colonial forms, usually exhibit alternation of gen- 

 eration. Salpa and Doliolum are the most common examples. 



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

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

 of which Aniphioxus (Branchiostoma lanceolatus) is the most common 

 representative. 



