328 M. Ussow's ZooIoqico-Emhri/oIof/i'cal Investigations. 



III. Tlie Boily-icaU. 



The envelope or wall of the body eonsists in all Tunieata 

 of two contiguous mantles — an outer {tunica externa) and an 

 inner one {tunica interna). 



In most of these animals (especially in the Chthonascidia) 

 the outer mantle consists of three layers : — rr, a peripheral 

 layer (sometimes wanting) of spiniform colls {C^nf/iifi) ; />, a 

 middle, more or less thick, gelatinous fundamental layer of 

 rather firm coalesced sheaths (the so-called '' test-cells''), pro- 

 duced from the epithelial cells of the memhrana granulosa of 

 the Graafian follicle ; and, c, a third layer, sometimes very 

 thin and scarcely perceptible (e. g. in Ascidia intestinalisj 

 canina] Sal pa), which is composed exclusively of long clastic 

 fibres, closely applied to the peripheral epithelial layer of the 

 inner mantle (but never coalesccnt therewith). The outer 

 mantle of the Tunieata is not developed as a product of 

 secretion of the epidermoidal cells of the inner mantle (Hert- 

 wig*, Arsenjewf), but by the multiplication and gi-owth of 

 the above-mentioned "test-cells" (KupferJ, A. lvoAvalevsky§), 

 which are at first arranged in a single layer between the yelk 

 and the vitelline membrane (chorion). The results of my in- 

 vestigations of the formation of the so-called test-cells agree 

 perfectly with those obtained by A. Kowalevsky i|. The yellow 

 corpuscles are in fact nothing but cells of the Graafian follicle, 

 ■which have arranged themselves in a single series round the 

 mature oviccll, and closed upon it before the formation of the 

 chorion. In the larvfe of the simple Ascidia, and in the em- 

 biyos of the SaJj)a' and Pgrosomataj the rudiment of the outer 

 mantle consists exclusively of radiating primitive yellow cells, 

 rapidly multiplying by division, and their intercellular sub- 

 stance. These cells, which put forth numerous processes 

 and not unfrequently change their position ("Wanderzellen," 

 Kolliker^), soon coalesce with their sheaths. The contractile 

 protoplasm of such cells gradually disappears (becomes ab- 

 sorbed?). The close network of coalesced thickened sheaths 

 thus forms the porous, vesicular, fundamental substance of the 

 inner mantle, always containing much water (especially in 



* Jen. Zeitschr. Band vii. 1872, p. 4G. 



t Berichte der Mosk. Gos. flir Aaturg. und Anthrop. Band x. 1872, 

 p. 86; Jahresb. liber die Anat. und Phyp. 1873, ^Vrt. Tunieata, p. 307. 



X Schultze's Arcbiv, Band vi. pp. 14!», loO. 



§ Entwickelungsf^escb. der cinf. Ascid. pp. 13 et seq. 



II Scbultze's Arcbiv, Band vii. pp. 10."} e( .•«•?. 



if Ann. Sci. Nat. tome v. p. 220; Mull. Arcb. 1852, p. 325; Scbvdtze's 

 Arcbiv, Band vi. p. 125. 



