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MOLLUSCA TUNICATA, 
DICHITONIDA. 
Inner tunic detached from the external one, and united only at 
tlie two orifices. The branchiae are large, equal, and spread 
on the central surface of the inner sac, and the branchial ori- 
fice has an inner membranaceous ring, or circle of tentacula. 
All the British Dichitonida are fixed, and the branchial and 
anal orifices are not opposite to each other. 
A. Body simple. 
a. Apertures furnished with four rays. The body sessile; branchife 
four on each side, the meshes uninterrupted. 
-- Pandocia. 
aa. Apertures with indistinct rays, or more than four, the tunic soft ; 
branchiae destitute of longitudinal folds. 
b. Body pedunculated. 
Clavellina. 
bb. Body sessile. 
c. Tunic and branchial cavity straight. 
Pirena. 
Ciona. 
cc. Tunic turned up at the base. 
Phallusia. 
AA. Body compound. 
a. Branchial orifice radiated. 
b. Branchial and anal orifices with six regular rays ; body sessile. 
Polyzona. 
bh. Branchial orifice only with six regular rays. 
c. Body pedunculated ; system single, circular, and terminal 
Sydneum. 
cc. Body sessile, polymorphous. 
Alpidium. 
aa. Branchial orifice simple. 
Botryllus. 
GgS 
