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 (ig. 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. 
Lnternal 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 maiter, 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, 2), 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. 
