TUNICATES. 693 
in different species of simple and compound Ascidi@, so that it may 
be safely concluded to be a common property of the animals of this 
class. The heart has an elongate form and does not exhibit any 
sudden expansion and contraction, but a sinuously progressive 
constricting and widening, which is similar in form to the peri- 
staltic motion of the intestinal canal. Besides larger vessels the 
blood moves in interspaces which have no proper walls. 
The respiratory organs are gills which are very different from 
those of the bivalve molluscs. In the genus Salpa the gill is 
extended as a longitudinal band running obliquely in the inner 
cavity of the tubular body, with many transverse grooves closely 
arranged side by side. In the Asczdie the respiratory organ forms 
a sac at the bottom of which the entrance to the cesophagus is 
situated. The very thin membrane, of which the sac consists, 
presents transverse and longitudinal stripes that cross each other at 
right angles like trellis-work, to which a net-work of blood-vessels 
corresponds. The gills, as well of the Salpe as of the Ascidic, 
are beset with vibratile cilia?. 
Male and female organs are, at least in the Ascidie, united in 
the same individual. Often they are intimately connected with each 
other and are only to be distinguished by microscopic examination 
of their content. Some Ascidiw, as Ascidia ampullaris V. BENED., 
have quite a double apparatus of sexual organs, which lies behind 
in the intestinal sac on each side of the intestinal canal. In some 
the oviduct is wanting, in others no vas deferens exists; a single, 
long, tortuous canal, which is present in the compound Ascidie, 
and is generally held to be the oviduct, ought, according to MILNE 
Epwarps, to be regarded as the vas deferens. In the Asetdie, 
besides the usual propagation by impregnated eggs, a multiplication 
by buds has also been detected”. 
1 Minne Epwarps and before him ListER admit openings like fissures between the 
quadrangular meshes by which the water is expelled from the branchial sac (MILNE 
Epwarps Observ. s. l. Asc. comp. pp. 17—20). It is difficult to determine, in speci- 
mens preserved in spirit, whether they are really openings or spaces closed by a trans- 
parent membrane, [HUXLEY says that in Pyrosoma the respiratory cavity communi- 
cates freely by means of apertures in the branchial network with the post-branchial or 
anal cavity. Phil. Trans. 1851, p. 581.] 
2 Compare for some other peculiarities, the notice of the different genera in the 
systematic arrangement, in which also the remarkable propagation of the Salpe is 
noticed. 
