STRUCTURE BODY-WALL, ETC. 



43 



ad. N| 



c7 



br.s' 



. end 



tissues both homogeneous and fibrous with cells, blood- 

 sinuses, and many muscle - bundles large and small running 

 circularly, longitudinally, and obliquely, and interlacing in all 

 directions (Fig. 18, 

 in). The muscles 

 are all formed of 

 very long fusiform 

 non- striped fibres. 

 The mantle in some 

 Ascidians is often 

 brilliantly pig- 

 mented red, yel- 

 low and opaque 

 white, the coloured 

 cells being exactly 

 like those found in 



the blood. 



ren. 

 The mantle 



forms two well- 

 marked siphons or oes.. 

 short wide tubes, 

 which lead in from 

 the branchial and 

 atrial apertures. brc 

 These are sur- 

 rounded by strong 

 sphincter muscles, 1 

 and are lined by the 

 invao~inated ecto- FIG. 18. Dissection of Ascidia, from right side, to show 



derm and tes The 

 one leads into the 

 branchial sac or 

 modified pharynx, 

 and the other into 

 the atrial or peribranchial cavity (see Fig. 18, and Fig. 19, p.lr). 

 Figs. 18 and 19 show the relations of the branchial and 

 peribranchial cavities to one another. The peribranchial cavity 



anatomy. , Anus ; At, atrial aperture ; Br, branchial 

 aperture ; br.s, br.s', branchial sac ; end, endostyle ; g.d, 

 genital ducts; yon, ovary; hyp, neural gland; hyp.d, 

 the duct leading to dorsal tubercle; m, mantle : n.;j, 

 ganglion ; oes, oesophagus ; p.br.c, peribranchial cavity; 

 ren, renal vesicles ; st, stomach ; t, test ; tn, tentacles ; 

 ty, typhlosole. 



1 These sphincters close the only openings through the tough test so effectually 

 that when collectors arc preserving Ascidians in alcohol it is advisable to make one 

 or more slits in the test to allow the sea-water to escape and the spirit to enter. 



