Tunicata. 193 



lig. 3) ; but sections show that although the test is thickened in this 

 region, the pad is really due more to a great increase in the bulk of 

 the mantle than of the test. In one specimen examined (fig. 4) the 

 thickest part of the test, at the posterior end, measured 5 mm., while 

 the mantle underneath it reached the extraordinary thickness of 

 10 mm. Over the rest of the body both test and mantle are thin. 

 The mantle though thin is muscular, and has distinct external 

 circular and internal longitudinal layers of fibres. The branchial 

 sac does not extend to the posterior end of the body, but leaves a 

 considerable cavity which is occupied by the alimentary canal. The 

 stomach and intestine thus come to lie posteriorly to the branchail 

 sac (fig. 7). The endostyle shows large glandular thickenings, forming 

 pad-like masses placed alternately at its sides (see fig. 6). There 

 are very broad shelf-like horizontal membranes on each transverse 

 vessel (fig. ;■)) ; otherwise the branchial sac is as shown in the 

 ' Challenger ' Report. There are two long gonads on each side of the 

 body, and in addition to these there are many irregular tag-like 

 " endocarps " (see fig. 8). 



ASCIDIAE COMPOSITAE. 

 Family P O L YC LINID AE. 



Tylobranchion antarcticvim. (Plate XX., figs. 1-6.) 



External appearance. — This is a small colony with a rounded 

 slightly lobed upper surface. Each lobe contains a small group of 

 two or three ascidiozooids placed vertically in the transparent test. 

 The colour is light grey, the test being lighter and the ascidiozooids 

 a little darker and showing opaque in the transparent mass. 



Test. — The lower part of the colony is a solid mass of test about 

 1 cm. in thickness, and covered with adhering and imbedded sand 

 grains, the upper part is soft and transparent. 



Mantle. — Thin with prominent muscle bands running longi- 

 tudinally. 



Branchial .sv/c— Large, with numerous stigmata. There are no 

 internal longitudinal bars, but the transverse vessels bear large 

 papillae which bifurcate at the free end (figs. 5 and 6). 



Ascidiozooid.— From o to 7 mm. in length, exclusive of the lung 

 post-abdomen or ectodermal tube. The visceral part (abdomen) is 







