152 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; e, eye-spot; 7, fin; 7, rod or noto- 
chord, the first faint indication of a backbone; ~ v, nerve-cord; g, 
gills; 2, hole out of which water passes from the gills; v, vent for refuse 
of food. 
hk. 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 (Scaéra). 
Class I]—PoucH-GILLED VERTEBRATES (Marsipo- 
branchit), 
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 cesophagus. 
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 (J¢yxine),—The hag is about a foot in length. 
The eyes are minute and under the skin. They are gen- 
