SECT. x. FLOATING ASCIDIANS. 225 



creatures vary in length from one to six inches : there- 

 fore they are not microscopic, yet their internal struc- 

 ture, which is similar to 

 that described, cannot be 

 determined without the aid 

 of that instrument. The 

 organ of hearing is a cap- 

 sule containing an otolite 

 and coloured spots placed 

 between the orifices; the 

 uppermost orifice or mouth 

 is surrounded by eight eye- 

 specks, and six Of a deep Fig. IC-J. Ascidia virginea. 



orange colour surround the 



lateral one, a nerve-centre between the two supplies 

 the animal with nerves. These Tunicata live on 

 diatoms and morsels of sea-weeds, and, like all the 

 fixed Ascidians, they show no external sign of vitality 

 except that of opening and shutting the two orifices. 

 More than fifty species of these solitary Ascidians in- 

 habit the British coasts from low-water mark to a 

 depth of more than one hundred fathoms. 



Pyrosomidce. 



The Pyrosomidse are floating compound Ascidians, 

 composed of innumerable individual animals united 

 side by side, and grouped in whorls so as to form a 

 hollow tube or cylinder open at one end only, and from 

 two to fourteen inches long, with a circumference varying 

 from half an inch to three inches. The inhalent orifices 

 of the component animals are all on the exterior of the 

 cylinder, while the exhalent orifices are all on its inside, 

 and the result of so many little currents of water dis- 

 charged into the cavity is to produce one general outflow 

 which impels the cylinder to float with its closed end 



VOL. II. O 



