APRIL — sept. 1858. J Order Radiata. 135 



In the case of the sea-sheats which are social the outer mantle of 

 of the various individuals, unites so that a common jelly-like cover- 

 ing is formed, in which the separate animals are placed and from 

 which they can easily be freed. 



Some of the Tunicata live either in colonies or separately each 

 are attached to the ground ; in this case their only activity is con- 

 fined to a lazy opening and closing of the openings in their mantle. 

 The free-livers can swim, and perform the feat in this remarkable 

 manner : — they suck in water with great activity, and then ejecting 

 it with equal force, the backstroke of the gushing water drives them 

 forward. In those cases where individuals are united in chains all 

 swallow and eject water simultaneously and in the Pyrosomidoe 

 where many individuals are united in one swimming mantle, the 

 mantle itself moves them on. 



The nervous system of the Tunicata consists of but one large 

 ganglion which is to be found on the back surface near the in- 

 gress opening. In the Salpce this ganglion, which is kettle-shaped, 

 bears on it the single eye, which is generally surrounded by a dark 

 red pigment. Other organs of the senses are completely wanting. 



The circulation of the blood is most clearly developed, and the 

 motion of it is always carried On by a bag-shaped heart which is 

 either placed near the breathing organs or at the bottom of the 

 body : from this heart are expelled streams of blood, which either 

 run in canals, or spread themselves in the interstices between the 

 organs. But the peculiar and remarkable circumstance is that in 

 all the Tunicata the direction of the stream of blood changes pe- 

 riodically. The heart makes a certain number of pulsations in 

 one direction, then it stands still suddenly, and recommences its 

 pulsations in the other direction. So it varies at irregular inter- 

 vals, often of several minutes, and in all the vessels one sees the 

 blood change, streaming first from left to right, then stop, and 

 then take the contrary direction. In the transparent Tunicata 

 one can easily convince oneself that these changes originate solely 

 in the heart : — one cannot therefore speak of veins and arteries as 

 the direction of the circulation is constantly changing. 



The breathing-apparatus is highly developed according to two 

 different types. In the Ascidise the water enters through the 



