330 M. Ussow’s Zoologico-Embryological Investigations. 
above-mentioned layer of test-cells, isolated both from the 
yelk and from the embryo. The only spot where the outer 
and inner mantles are more closely united is that branch of 
the main ventral artery-vein (“ s¢nus dorsalis,” Milne-Edw.) 
through which the network of blood-vessels of the outer 
mantle is connected with the principal blood-system of the 
Tunicata, the chamberless cardiac tube. 
The inner mantle, or dermo-muscular sac, of the Tunicata 
consists of a transparent porous and fibrous substance, formed 
by the coalescence of the stellate cells. In this layer are the 
muscular bundles and the variously intercrossing fibres of the 
connective tissue. On the side towards the branchial sac*, and 
on that towards the outer mantle, this layer is covered with 
pavement epithelium. ‘The whole inner mantle is developed, as 
has been proved (by A. Kowalevsky, Kupfer, and others), from 
the cells of the upper germ-lamella, and consequently, as 
regards its mode of formation, corresponds to the epidermoidal 
coverings of all other animals. 
IV. The Blood-vascular System. 
The exact investigation of the blood-vascular system in the 
simple Ascidia (Ascidia intestinalis, canina, mammillata) and 
the Salpe (Salpa maxima, fusiformis, pinnata, bicaudata) has 
proved to me that the blood of these animals circulates in a 
system of closed vessels. It is possible that it is only the 
small size of many compound Ascidiev, Pyrosomata, Cyclo- 
myarie, and Appendicularie that has prevented the detection 
of similar vessels furnished with walls in these animals also J. 
The walls of the vessels consist of a single layer of flat 
rhomboidal cells. The results obtained by me with regard to 
the distribution of the blood-vascular system in the inner 
mantle, in the branchial sac, and in many other parts of the 
body, agree perfectly with the investigations made by Milne- 
Edwards | and N. Wagner§ on this matter. Among the 
* In all the Tunicata investigated by me I have found on the inner 
mantle, besides the external adherent epithelial layer, a second such layer 
on the side towards the branchial sac, See Leuck. Zool. Unters. Heft ii. 
. 13. 
Z + The very general belief in the lacunar system of the Tunicata 
(Gegenbaur, Vergl. Anat. 2te Aufl. pp. 245, 244) does not seem to be 
satisfactorily supported by facts. Payen’s opinion (Joc. eit. p. 283) that 
the vascular system is atrophied in the adult Pyrosomata is not supported 
by my investigations, as the main vessels, at least, are always furnished 
with walls. 
ick Mém, de l’Acad. de Paris, tome xviii, Régne Anim. Mollusques, 
§ Mél. biol. de Acad, de St, Pétersh. tome yi. 1866, p. 11 e¢ seg. 
