J 
YOUNG ASCIDIAN 
c. Baan Pora 
YOUNG AMPHIOKLS 
ZOOLOGY. 
39 
shut off from the general cavity of the body: a great 
advance in structure as compared to that of the Anem- 
one and Jelly-fish, The Bryozoa are usually classed 
with the Clams and Oysters (Mollusca); but, from their de- 
velopment from worm-like embryos, they are more justly 
considered as a group of worms. This view of the po- 
sition of the Bryozoa is confirmed by the recent discov- 
eries of the worm-like development of the Brachiopoda, 
the first class of the Mollusca, The Bryozoa are probably 
the root of the Mollusca, and the connecting link between 
them and the Worms. The Tunicata, the other division 
of the Sac-worms, are so called from the animals repre- 
senting them being inclosed in a bag or tunic. They are 
a very important group, as showing probably that point 
where the stem of the Fishes originated. The young As- 
cidian (Fig. 38, 4), one of the Tunicata, resembles a tad- 
pole, and in this condition has quite as much of a back- 
bone (Fig. 38, C) as the Amphioxus (Fig. 39, 40, C), the 
simplest vertebrate known. The Ascidian, when mature, is 
like a double-necked vase. The arrangement of the ner- 
vous system in the Tunicata differs from that of the Bryo- 
zoa, and serves as a distinguishing mark. The curious 
worm Sagitta (Fig. 36), the only representative of the 
Chetognathi, has certain affinities with the thread-worm as 
well as with the simplest of the vertebrata; it is therefore 
placed between the two. 
The Ceelenterata are characterized by the want of speci- 
alized structures; that division of labor, so conspicuous 
in the higher animals, begins to be seen in the Worms, 
the digestive system in them being more or less developed, 
together with a rudimentary heart, respiratory and excre- 
tory apparatus, and the elements of a nervous system. 
The Tree of Worms is essentially an intermediate one,—its 
roots intimately connected with the simplest forms of life, 
Gregarinze, Infusoria, etc.,—its branches expanding into the 
