136 
ZOOLOGY, 
the finger-bones or iilialanges^ 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 tihia 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 of 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 MarsipohrancMi, 
4. Swimming by fins; with gills; a movable under 
jaw Pisces, 
5. Amphibious; true limbs and lungs ; skin smooth, 
no scales, no claws BatracMa. 
6. Claws and scales present Bejptilia, 
7. Body covered with feathers ; fore-limbs forming 
wings Ams, 
8. Body covered with hair ; suckling their young. . .Mammalia, 
Cla&s I. — Tui^iCATA {Ascidians, Sea-squirts). 
General Characters of Tunicates. — While the Tunicates 
were formerly supposed by some to be moUusks, 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 we have just noticed, 
while all other Ascidians which undergo 
Jjps^ a metamorphosis lose their tails, notocord, 
Fig. 177.— Moiguia. and ncrvous cord, and degenerate into or- 
An Ascidian. a . -, . 
dmary 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 
