TUNICATES. 693 



in different species of simple and compound Ascidice, 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 Ascidice the respiratory organ forms 

 a sac at the bottom of which the entrance to the oesophagus 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 Salpce as of the Ascidice, 

 are beset with vibratile cilia J . 



Male and female organs are, at least in the Ascidice, 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 Ascidice, 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 Ascidice, 

 and is generally held to be the oviduct, ought, according to MILNE 

 EDWARDS, to be regarded as the vas deferens. In the Ascidice, 

 besides the usual propagation by impregnated eggs, a multiplication 

 by buds has also been detected 2 . 



1 MILNE EDWAKDS and before him LISTEE admit openings like fissures between the 

 quadrangular meshes by which the water is expelled from the branchial sac (MlLNE 

 EDWAEDS Observ. s. I. Asc. cornp. 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 Salpce is 

 noticed. 



