TUNICATA. 
85 
vertical section roughly shows the animal to be formed of three 
concentric sacs (Figs. 2, 3). The outermost, which is tough and 
Diagrammatic dissection of A. mentula. 
at, atrial orifice; br, branchial orifice; a, anal orifice; brs, branchial sac; 
dl, dorsal lamina ; end, endostyle ; m, mantle ; ng, nerve ganglion ; oea, 
orifice of gullet ; pbr, peribranchial cavity ; st, stomach ; t, test ; tn, 
tentacles. 
(After Herdman : Tunicata, Encyc. Britannica.) 
membranous, is called the Test or Tunic, the whole group owing 
its name to the presence of this protective covering. 
The middle sac, termed the Mantle, which almost corresponds 
in shape to the outer, is composed of connective tissue, muscle-fibres, 
blood-vessels, etc. ; in spirit specimens, the mantle is shrunk away 
from the test except at the orifices and at a point behind, where 
vessels enter the test. 
The innermost or Branchial Sac is attached behind the branchial 
orifice and along the ventral edge, but otherwise hangs free in the 
