layer of the test (fig. 9, t.k.)', in some Ascidians they 

 prohably form an accessory organ of respiration. In the 

 adult Ascidia the vessels enter the test by a double trunk 

 ventrally, near the p )sterior end of the left side (see PI. 

 III., fig. 10). 



Ectoderm and Body-wall. 



Inside the test, and lying between its inner surface and 

 the muscular body-wall ("mantle"), is a very delicate 

 membrane, formed entirely of small cubical or more flat- 

 tened cells (PL II., fig, 5, ec), with delicate "secreting 

 processes " projecting into the test. This is the Ecto- 

 derm, the outermost complete layer of cells in the body ; 

 and the test lying over it has been produced as a sort of 

 gelatinous exudation upon the surface of the ectodermal 

 cells, aided by the numerous mesoderm cells which have 

 migrated into it, and which give it the appearance of a 

 connective tissue. Besides the ordinary epithelial cells, a 

 few gland cells and pigment cells may be found in the 

 ectoderm. The ectoderm is turned in for a short distance 

 at the branchial aperture (mouth) and atrial aperture 

 (cloacal), as a short stomodteum and proctodaeum, lined 

 in each case by a delicate prolongation of the test (PI. II., 

 fig. 4, Br. and At.). 



Inside the ectoderm lies a thicker layer, the so-called 

 " Mantle " or body-wall (paiietal mesoderm), containing a 

 large number of muscles, which run some along the length 

 of the body, and others across it, so that they form a rude 

 interlacing net-work (PL II., fig. 4, m.), which is much 

 more strongly developed on the right than on the left side 

 of the body. The shape of the body can be changed, to a 

 slight extent, by means of these muscles in the body-wall. 

 When the Ascidian is killed by re-agents it is generally 

 found that the muscles have contracted and drawn the 



