136 ZOOLOGY. 



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

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

 tached the hind limbs, consisting each of &, 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 Tunicata. 



2. No skull or brain; blood colorless Leptocardii. 



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



of purse like gills Marsipobranchii. 



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 Reptilia. 



7. Body covered with feathers ; fore-limbs forming 



wings Aves. 



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



Class I. — Tunicata (A, scidians, Sea-squirt*). 



General Characters of Tunicates. — While the Tunicates 

 Avere formerly supposed by some to be mollusks, and by 

 others worms, they have been found to possess in the lar- 

 val stage a notocord, above which lies a 

 rudimentary brain, with a spinal cord, and 

 even spinal nerves. A tadpole-like form 

 (Appendicularia) retains the fundamental 

 vertebrate features Ave have just noticed, 

 -l|fl while all other Ascidians which undergo 

 pls^ a metamorphosis lose their tails, notocord, 



fiq. it?.— Moiguia. and nervous cord, and degenerate into or- 

 An Ascidian. n . , . -, • 



dinary Ascidians. 



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 Botryllus grows on the leaves 



