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 are common on sea- weeds and sub- 

 merged stones. These are Botryllidce. We 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 Asciclia, from the Greek word do-Kos, 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 see the orifices periodically 

 opened wide, and suddenly contracted to a point ; and by 

 careful observation we may detect the entering and out- 

 going currents of water that pass through these apertures. 



There are some species which, though not strictly Com- 

 pound, are aggregated together in a highly curious manner. 

 Such are the Salpce, which are found swimming in the free 

 ocean; sometimes solitary, sometimes united into long- 

 flexible chains of transparent animals, which swim with 



