1 52 BACKBONED ANIMALS. 



bling a larval ascidian. They breathe by taking in water 

 at the mouth, m, that passes through the gill-slits, g, oxy- 

 genating the blood, then passing out through an opening, 



FIG. 190. The lancelet. m, mouth ; , eye-spot ; /, fin ; r, rod or noto- 

 chord, the first faint indication of a backbone ; n v, nerve-cord ; g, 

 gills ; ^, hole out of which water passes from the gills ; v, vent for refuse 

 of food. 



h. Compare this with the respiration of ascidians (page 

 146). The common lancelet lives in the sand in the 

 shallow water of our coast, from Virginia to southern 

 Florida. 



An Australian lancelet has a high dorsal fin, and about 

 Zamboanga one is found living in a sea-cucumber (Seabra). 



Class II. POUCH-GILLED VERTEBRATES (Marsipo- 

 branchii). 



General Characteristics. The animals of this class are 

 worm-like in appearance. They have no jaws, the mouth 

 being adapted for sucking. The respiratory organs (Fig. 

 191) are pouch-like cells or cavities, which open externally 

 by seven small holes in the lamprey, and internally con- 

 nect with the mouth and a cavity beneath the oesophagus. 

 The nasal aperture that, in the higher Vertebrates, is 

 paired, is here single, in the hag connecting with a sac 

 that leads to the mouth ; but in the lamprey there is no 

 such connection. 



Hag (Afyxine). The hag is about a foot in length. 

 The eyes are minute and under the skin. They are gen- 



