190 INVERTEBRATE ANIMALS. 
organism consists of a number of zodids, produced by continu- 
ous budding, and connected together by a common tube, through 
which the blood circulates. In the so-called “compound” 
Ascidians the tests are fused together into a common gelatinous 
mass, in which the individuals are embedded in groups. Some 
of the 7umnicata are oceanic—that is to say, are found floating 
or swimming at the surface of the open ocean—and some exhibit 
the phenomenon of phosphorescence. 
In the foregoing description it has been found impossible to convey 
even the most elementary outline of the anatomy of a Tunicate without 
having recourse to technical terms. There still remain a few points of 
homology which should be mentioned. In the foregoing, the so-called 
“oral aperture” of the animal has been regarded as truly the mouth, 
this being the simplest view, and the one held by Professor Huxley. 
Upon this view the ‘‘respiratory sac,” into which the mouth opens, 
must be regarded as a greatly-developed pharynx (z.¢., the upper portion 
of the alimentary tube). Similarly, on this view, the ower aperture of 
the respiratory sac will have to be regarded as the opening of the gadé/er. 
By Professor Allman, again, the respiratory sac is looked upon as 
formed by a great modification of organs corresponding to the ciliated 
tentacles of the Polyzoa, so that the /ower aperture of the respiratory sac 
is the true mouth. Lastly, by Professor Rolleston the respiratory sac 
is looked upon as corresponding to the gills of the bivalve shell-fish 
(Lamellibranchiata), and the oral and atrial apertures are regarded as 
corresponding to the ‘‘ respiratory siphons” of these same animals. On 
this view the ower aperture of the respiratory sac is again looked upon 
as the true south. ‘The question cannot be regarded as settled, and 
Huxley’s view has been here adopted merely as being the most readily 
intelligible to learners. 
Cuass III. BRACHIOPODA.—The members of this class are 
little known to the general public, being all marine, often in- 
habiting considerable depths in the sea, and being much more 
abundantly represented by fossil forms than by living examples. 
They are often placed with the ordinary Bivalve shell-fish 
(Lamellibranchiata), in consequence of their universally pos- 
sessing a shell composed of two pieces or valves (fig. 98), but 
they are really of a much lower organisation. In their essential 
structure they show many points of affinity to the Polyzoa, but 
they are always simple animals, never forming colonies, and 
they always have a bivalve shell. The two pieces of which the 
shell is composed are always placed one in front and one be- 
hind, so that they are “ventral” and “dorsal,” and not “right ” 
