136 ZOOLOGY. 



the finger-bones or jihalanges, the single row of phalanges 

 forming the digit (finger or toe). To the pelvis are at- 

 tached the hind limbs, consisting each of a femur or thigh, 

 which is succeeded by the tibia and fibula (shank-bones), 

 which are followed by the tarsal and metatarsal bones 

 (ankle-bones), and by the phalanges or bones forming the 



toes. 



Classes op Vertebrates. 



1. Young with a nervous and dorsal cord Tiinicata. 



3. No skull or brain; blood colorless Leptocardii. 



3. Notocord persistent; no jaw-bones; six to ten pairs 



of purse lik^ gills Marsipohranchii. 



4. Swimming by fins; with gills; a movable under 



jaw Pisces. 



5. Amphibious; true limbs and lungs ; skin smooth, 



no scales, no claws Batracliia. 



6. Claws and scales present Replilia. 



7. Body covered with feathers ; fore-limbs forming 



wings Ares. 



8. Body covered with hair ; suckling their young. . .Mammalia. 



Cla.'js I. — TuNicATA {Ascidiaiis, Sea-squirts). 



General Characters of Tunicates. — While the Timicates 

 were formerly supposed by some to be mollusks, and by 

 others worms, they have been found to j^ossess in the lar- 

 val stage a notocord, above which lies a 

 rudimentary brain, with a spinal cord, and 

 i^ even spinal nerves. A tadpole-like form 

 {Appendicularia) retains the fundamental 

 S>^l vertebrate features we have iust noticed, 

 L'fi;'; • v;'4®%F wliile all other Ascidians which undergo 

 ■'^^^p^ a metamorphosis lose their tails, notocord, 

 Fig. 177.— Moiguia. and uervous cord, and degenerate into or- 



An Ascidian. t . . ,. 



dmary Asciitians. 

 The Ascidians are common just below low-water mark, 

 either hidden in masses of mussels, attached to the rocks 

 under sea-weeds, or the compound species may be found 

 forming bright-colored masses on the piles of wharves and 

 bridges, while the star-like Botri/llus grows on the leaves 



