56 SQUIRTS, POLYPS, AND JELLY-FISHES. 



a peculiar sieve-like chamber — whose walls are 

 made up of delicate vessels — into which food parti- 

 cles are carried, and through which the admitted 

 water passes into a second chamber, 

 which also receives the alimentary 

 canal. From this second chamber 

 the water, which bathes the blood- 

 vessels forming the meshes of the 

 sieve-like respiratory sac, is expelled 

 through the second chimney-pot 

 opening, and thus a constant circu- 



STRUCTURE of a TUNI- 1 , • ■ 1 , A 1 "LI 



CATE. M, mouth ; s, lation IS kept up. A remarkable 

 respiratory sac ; A, fact connected With the Circulation 



atrial chamber. /. ,, - • ^ • ^^ i. ^^ i i 



or this animal is that the heart, 

 which lies near the base of the respiratory sac, after 

 beating a short time suddenly stops, and that with 

 each renewal of action the direction of the blood- 

 current is reversed. From the peculiar external 

 tunic which encases the animal, the group to which 

 the squirt belongs has been designated the Tuni- 

 cata. 



A considerably larger form than Molgula is that 

 which has received from fishermen the name of 

 ' sea-peach' (Cynthia), in allusion to the similarity 

 in form and coloring existing between it and the 

 peach. Other species of nearly identical structure 

 and habit are the ' sea-pears' (Boltenia), which are 

 supported on long slender stalks, measuring as 

 much as a foot or more in length. These are almost 

 invariably covered with foreign associations of plant 

 and animal matter, presenting a coarse and untidy 

 appearance. 



