INTRODUCTION. XV. 



certain cell modifications may produce very various colours, 

 appearances, and histological conditions of the test in 

 different Ascidians. Finally, the test is notable amongst 

 animal tissues for containing " tunicine," or animal cellulose. 

 The branchial sac (figs. 2, 3, 4) is the dominant organ in the 

 anatomy of an Ascidiau, and shows almost endless variations 

 in structure, many of which are made use of in classification 

 and in the diagnosis of species. The wall of the branchial 

 sac is penetrated by a large number of channels, through 

 which blood flows. These form the boundaries of the 

 stigmata (fig. 3, sg.) through which the water passes from 

 the branchial to the atrial cavity. The chief vessels or blood 

 channels are: the "transverse' (PI. A, fig. 3, /.), which 

 run dorso-ventrally round the sides of the sac, the " inter- 

 stigrnatic* or fine longitudinal (fig. 3, /..), and the stout 

 internal longitudinal bars (/././>.) which run antero-posteriorly 

 and with the transverse vessels form prominent meshes 

 containing each a certain number of stigmata. The diagram 

 (fig. 2) shows how the atrium surrounds the branchial sac 

 on all sides, except the ventral, where the wall of the sac 

 becomes continuous with the mantle. 



The branchial sac is very large in most Ascidians, and 

 extends nearly to the posterior end of the body, while the 

 rest of the alimentary canal lies (in Ascidia) upon its left 

 side. The opening of the pharynx (branchial sac) into the 

 oesophagus is placed far back on the dorsal edge, and is 

 connected with the front of the sac (mouth) by a prominent 

 ridge or fold, the dorsal lamina (d.L, figs. 2, 6, 7). 



Along the opposite or ventral edge of the sac runs a 

 conspicuous groove with thickened lips, and containing 

 glandular cells for the secretion of mucus. This is the 

 endostyle. Food particles entering the mouth first pass 

 through a circle of tentacles (simple in Ascidia, branched 

 in many other Ascidians) attached round the base of the 



