VERTEBRATA. 
309 
Division I.—Urochordata. 
Class I.— Tunicata. 
The Tunicates form a small and singular group of animals 
having relations with the worms on the one hand and with 
the Vertebrates on the other. The most common forms 
(the solitary Ascidians ) are 
enclosed in a leathery, elastic 
bag, one end of which is fast¬ 
ened to the rocks, while the 
other has two orifices, for the 
inlet and exit of a current of 
water for nutrition and res¬ 
piration. They are without 
head, feet, arms, or shell. In¬ 
deed, few animals seem more Fio. 280 .— An Ascidian. 
helpless and apathetic than these apparently shapeless be¬ 
ings. The tubular heart exhibits the curious phenomenon 
of reversing its action at brief intervals, so that the blood 
oscillates backward and forward in the 
same vessels. Another peculiarity is the 
presence of cellulose in the skin. The 
water is drawn by cilia into a branchial 
sac, an enlargement of the first part of 
the intestine, whence it escapes through 
openings in the sides, to the excurrent ori¬ 
fice, while the particles of food drawn in 
with the water are retained and passed 
into the intestine. The larva is active, 
swimming by means of a long tail. It 
looks like a tadpole, and has a notochord 
and a nervous system closely resembling 
those of a Vertebrate. Afterwards it at- 
ltt.iOI.*—X/lttglttHi UiUf ^ ^ 
pieAscidian: b,s, bran- taches itself by the head, the tail is ab- 
chial sac ; n, nervous . 
ganglion;s,stomach ; i, sorbed, and the nervous system is re- 
uy t ro t rgan;\heart dUC ’ duced to a single small ganglion. Thus 
