ORDER NUDA. 129 
sequent period, a fact which is in direct opposition to this 
opinion. 
Their gills, as in the ascidiz, form a large sac, traversed 
by the aliment before it arrives at the mouth; their principal 
ganglion is also situated between the mouth and the anus; a 
nearly similar disposition obtains with respect to the viscera 
and ovary. 
Notwithstanding this, some of them, like the biphora, have 
an opening at each extremity; such is the 
BOTRYLLUS, Gerin., 
Of an oval form, fixed on various bodies, and united by tens 
or twelves like the rays of a star. The branchial orifices are 
at the external extremities of these rays, and the anus termi- 
nates in a common cavity, which is in the centre of the star. 
If an orifice be irritated, but a single animal contracts; if the 
centre be touched, they all contract. ‘These very small ani- 
mals attach themselves to ascidie, fuci, &c. (Botryllus Stel- 
latus, Gertn., &c.) 
In some particular species three or four stars appeared to 
be piled one upon the other. (Botryllus conglomeratus, 
Geertn.) 
Pyrosoma, Péron., 
The pyrosome unite in great numbers, forming a large 
hollow cylinder, open at one end and closed at the other, 
which swims in the ocean by the combined contraction and 
dilatation of all the animals which compose it. The latter 
terminate in a point on the exterior, so that the whole ex- 
ternal surface of the tube is bristled with them ; the branchial 
orifices are pierced near these points, and the anus opens in 
the internal cavity of the cylinder. A pyrosoma may be thus 
compared to a great number of stars of botrylli strung toge- 
ther, the whole of which is moveable. 
VOL, XII. K 
