SECT. X. 



TUNIC AT A, OR ASCIDIANS. 



223 



and attached by a foot-stalk to a silvery stem like a 

 thread which stretches over the surface of stones, or 

 twines round the stalks of sea- weeds ; and as the stem 

 increases in length, buds spring from it, which in time 

 come to maturity, so that the silvery thread connects 

 a large .community ; but, though thus connected, every 

 member has its own individuality. Fig. 161 repre- 

 sents one of these transparent individuals very highly 

 magnified. 



The respiratory sac 

 occupies the upper 

 part of the body. It 

 is perforated by four 

 rows of narrow slits, 

 edged with cilia, 

 whose vibrations are 

 distinctly seen 

 through the transpa- 

 rent tunic of the little 

 animal. A portion of 

 the water which is 

 drawn by the cilia 

 into the upper orifice 

 or mouth, passes into 

 the respiratory sac, 

 escapes through the narrow slits into the space between 

 the sac and the tunic, and from thence into the stomach, 

 where any particles of food it may bring are digested, 

 and the refuse is carried by the current through the 

 intestinal canal, and ejected at the lateral orifice. 



The heart is a long multiform muscle, attached to the 

 respiratory sac, from whence capillary vessels spread over 

 that sac and throughout the body. The pulsations of 

 the heart drive the blood through the general system, 

 and bring it back to the heart again. After a time the 

 pulses of the heart become faint, and the blood ceases 



Fig. 161. Highly magnified Perophora. 



