256 COMPARATIVE ZOOLOGY. 
and a well-defined nervous system. The cells of a group 
never have connection with a common tube, as in Ceelen- 
terates. There are both marine and fresh-water species. 
Cuass I].—Tunicata. 
The Tunicates are either single or compound, and are 
found in all seas. The most common form (the solitary 
Ascidians) are inclosed in a leathery, elastic bag, one end 
of which is fastened to the rocks, while the other has two 
orifices, for the inlet and exit of a current of water for 
nutrition and respiration. They are without head, feet, 
arms, or shell. Indeed, few animals seem more helpless 
and apathetic than these ap- 
parently shapeless beings. 
The tubular heart exhibits 
the curious phenomenon of 
reversing its action at brief 
intervals, so that the blood 
oscillates backward and for- 
ward in the same vessels, as 
was supposed to be the case 
ee in the human system before 
the time of Harvey. Another peculiarity is the presence 
of cellulose in the skin. While the Ascidians are fixed 
and single, the Salpians are free, and alternately single 
and social. They are usnally seen swimming in long 
chains (the offspring of one individual). Each member of 
this colony produces solitary young, which are unlike their 
parents, and these again give birth to aggregated forms. 
Crass II].—Brachiopoda. 
These Mollusks have a bivalve shell, the valves being 
applied to the dorsal and ventral sides of the body. The 
valves are unequal, the ventral being usually larger, and 
more conyex; but they are symmetrical, ¢. e., a vertical 
