236 LIFE, IN ITS INTERMEDIATE FORMS. 
The breathing cilia are, in short, transferred from the pro- 
jecting tentacles to the interior of the body. There is, 
moreover, in the Tunicata a pulsating heart, and also a 
circulation of blood. 
In many families of this Class the individual animals 
are aggregated together like those we have just described, 
having a common corporate, as well as an individual life. 
Such are the brilliantly-coloured masses, like stars set in 
coloured jelly, that ai-e common on sea-weeds and sub- 
merged stones. These are Botryllidm. IVe know of no 
example of an isolated Polyzoan ; they are all compound 
animals, though in a few cases the cells stand up singly 
and remotely, from the common uniting root-thread ; but 
in the Tunicata we find many examples of single life. Of 
this sort are the strange uncouth creatures that are so 
abundantly brought up by the dredger from the sea- 
bottom, attached to stones and old shells, and resembling 
a bag of tough leathery skin, with two orifices, and hence 
called Ascidia, from the Greek word uo-ko?, a leather bottle. 
Some of these are large, rough with irregular lumps and 
depressions, and opaque ; others are smaller, smooth, 
pellucid, and brilliantly coloured. If they are plunged 
into a vessel of sea-water we sec the orifices periodically 
opened wide, and suddenly contracted to a point ; and by 
careful observation wc may detect the entering and out- 
going currents of water that pass through these apertures. 
There arc some species which, though not strictly com- 
pound, are aggregated together in a highly curious manner. 
Such are the Salpai, which are found swimming in the free 
ocean; sometimes solitary, sometimes united into long 
flexible chains of transparent animals, which swim with 
