330 M. Ussow's Zoohgico-Enibryological Invest I'qnf tons. 



above-mentioned layer of test-cells, isolated both fnini 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^inus Jorsalisy^^ MWnc-FAw.) 

 thron,<2^h which the network of blood-vessels of the outer 

 mantle is comiected with the principal blootl-system of the 

 Tunicata, the chambcrless cardiac tube. 



The inner mantle, or dcrmo-niuscular sac, of the Tunicata 

 consists of a transparent jx^rous 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, con-esponds to tlie epidermoidal 

 coverings of all other animals. 



IV. T7ie Blood-vascular System. 



The exact investigation of the blood-vascular system in the 

 simple Ascidia [Ascidia intestinalis^ canina^ mammillata) and 

 the Salpa; [Saljya maxima^ fusifonnis^ pinnata^ hicaudata) 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 Asci'dio'j Pyrosomata^ Cycle- 

 7nyar{a>j and Appendicular ioi that has ])revented the detection 

 of similar vessels furnished with walls in these animals alsof. 

 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 X 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 sucli layer 

 on the side towards the branchial sac. See Leuck. Zool. Unters. Ileftii. 

 p. 13. 



t The very general belief in the lacunar system of the Tunicata 

 (Gegenbaur, Vergl. Anat. 2te Aufl. pp. 24.3, 244) does not seeui to be 

 satisfactorily supported by facts. Payen's opinion (/oc. cit. p. 283) that 

 the vascular system is atrophied in the adult I'jfrosomdta is not supported 

 by my investigations, as the main vessels, at least, are always furnished 

 with walls. 



t M^m. de TAcad. de Paris, tome xviii. KSgne Anim. Mollusq^uea, 

 1842, 



§ M^l. bid. de I'Acad. de St Petcrab. tome vi. 1866, p. 11 e< seq. 



