246 ANIMAL ACTIVITIES. 
Embryology. The sea-squirts, called also ascidians 
and tunicates, were once classified as belonging to the 
Mollusca, and later to the Vermes. Zodlogists now 
class these animals with the Chordata, because a study 
of their development shows decided vertebrate charac- 
teristics, which disappear with maturity. The larval 
forms of some of these animals resemble the tadpoles 
of frogs, showing the notochord and the gill-slits 
so characteristic of vertebrate animals. Here it is the 
Fic. 199.—The Archzopteryx. 
larval stage alone which shows the true relationship of 
the animal. 
As already pointed out, barnacles and fish-lice do 
not show in adult life the characteristics which would 
place them among Crustacea. In many other cases of 
similar degeneracy the true kinship can be found only 
by referring to embryonic stages of growth. 
Another simple animal classed with the Chordata is 
the lancelet, or amphioxus, a headless, semi-trans- 
parent, boneless creature, which resembles the early 
