226 GENERAL ZOOLOGY 



Completely change their form, and have often during evolu- 

 tion come to serve entirely different functions from those 

 for which they were originally used. Such changes have 

 usually gone with modifications in modes of life. Though 

 all the primitive chordates are largely aquatic, the special- 

 ized forms are nearly all terrestrial and hence fitted to 

 breathe air. One of the most interesting features of the 

 past evolution of the vertebrates has been their migration 

 from water to land. 



The phylum Chordata is divided into four subphyla. 



Subphylum 1. Enteropneusta. This group includes cer- 

 tain worm-like animals. Dolichoglossus lives buried in the 

 mud along the seashore (Fig. 90, /). Its chorda is small 

 and contained within a swollen proboscis which projects 

 above the mouth. The body is divided into three regions 

 proboscis, collar, and trunk. A large number of paired 

 gill slits connect the pharynx with the exterior. Dolicho- 

 tilossus extracts its food from mud which is forced into the 

 mouth at the front of the collar, passes through the straight 

 enteron, and leaves the body through the anus at the poste- 

 rior end. 



The larvae of some of the Enteropneusta are tornarise, 

 which are much like the larvae of certain echinoderms, and 

 resemble a trochophore (page 190) in general structure. 

 This has led some zoologists to believe that chordates arose 

 from a remote ancestor which resembled a trochophore, and 

 that there is relationship between echinoderms, annelids, 

 molluscs'/ and other animals originating from a trochophore- 

 like larva. 



Subphylum 2. Tunicata. The sea-squirts (Fig. 90, g, h) 

 are degenerate animals which grow attached to piles and 

 other objects in the ocean. They are covered by tough 

 " tunics," through which there are two openings for the 

 entrance and exit of water. When taken from the ocean 

 two little streams of water are ejected from the body; 

 hence the common name, " sea-squirt." In feeding, water, 

 is strained through a porous pharynx, which catches food 



