TUNICATA. 25 



whole internal surface of which is covered with 

 \'ibrating cilia, while the sac itself is composed of 

 a tissue of blood-vessels, crossing each other at 

 right angles, bo as to form a muslin-like fabric of 

 exquisite fineness. 



In order to understand the relation of this respi- 

 ratory sac with the breathing apparatus of the 

 PoLYZOA, it seems to me that we have only to 

 suppose the tentacles (which are, doubtless, the 

 blood-vessels of the latter) turned into the gullet, 

 and spread over its interior surface, giving off 

 a number of cross branches, uniting the whole into 

 a network ; — and we have essentially the sac of the 

 Ascidia. 



Few microscopic spectacles are more interesting 

 than the sight of the circulation of the blood along 

 the network of this exquisite tissue, and that of the 

 ciliary waves by which the circulating fluid is 

 revivified. In the transparent species, such as 

 Clavelina, Pei^ophora, &c., both phenomena are 

 seen to great advantage. The effect of the reticu- 

 lation is to divide the whole surface of the sac into 

 oblong or oval spaces, set in regular rows ; which 

 spaces, being formed by the modified tentacles with 

 their ramifications, bear the cilia on their circum- 

 ferences. The optical effect of vibrating cilia is, 

 as is well known, an appearance of dark teeth 

 chasing each other ; and in this case we seem to 

 see a vast number of oblong or oval rings, set in 

 orderly arrangement, each composed of a circular 

 series of these running teeth, like the cogs on the 

 crown-wheel of a watch, chasing each other round 

 and round, with an even, moderately rapid, inces- 

 sant pursuit. 



At the same time, between the oval spaces — that 



