146 SEA-SQUIRTS. 



again, swimming free, singly, in chains upon the surface, or 

 attached to the bottom. Fig. 185, B, represents a com- 

 mon form of a fixed ascidian or tunicate resembling a jar 

 or bottle with two mouths. 



Covering. In the simple ascidians (Fig. 185) the body 

 seems inclosed in two coats, a leathery outer one called 

 the test, composed mainly of cellulose, a substance usually 

 found in plants, and an inner muscular one. 



Internal Structure. The sac is provided with two 

 openings, generally surrounded by short incurved tenta- 

 cles ; one is the mouth, m, and the other for the passage 

 of rejected matter, the fertilized eggs, etc. The mouth is 

 generally the upper opening, and leads into a chamber 

 called the respiratory sac, g, whose walls are perforated 

 with a network of ciliated openings. An orifice in the 

 bottom of the sac leads to the gullet that connects with 

 the stomach and intestine, the latter bending and finally 

 leading to a chamber connected with the ex-current or 

 atrial orifice. The liver is large and of a vivid green 

 hue ; the ovaries yellow. When the tunicate is handled, 

 water is ejected from both openings : hence the name 

 sea-squirt. 



Circulation. The heart (Fig. 185, /i), by the beating of 

 which circulation is effected, is a straight, tubular, mus- 

 cular organ, open at both ends. For a certain number of 

 times in some species, the blood is thrown one way, then 

 the action is reversed and it is propelled in the opposite 

 direction, so flowing alternately. 



Respiration. The network that we have seen in the 

 branchial chamber is traversed by blood-vessels that here 

 are brought in contact with water that is wafted along by 

 the cilia ; the blood takes up oxygen, and so is purified. 

 As food is also brought in with the water, the sea-squirt 

 breathes and obtains food by the same action.* 



* Compare this with the account of the oyster, on page 52. 



