STRUCTURE BODY-WALL, ETC. 



43 



3"- 



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, 

 m). 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. 



re 

 The mantle 



forms two well- 

 marked siphons or ". 

 short wide tubes, 

 which lead in from 



r, 



the branchial and 

 atrial apertures. ^^. 

 These are sur- 

 rounded by strong 

 sphincter muscles,^ 

 and are lined by the 

 invaginated ecto- 

 derm and test. The 

 one leads into the 

 branchial sac or 

 modified pharynx, 

 and the other into 





.end. 



--tjr 



....st 



FlQ. 18.— Dissection of Ascidia, from riglit side, to show 

 anatomy, a, Anus ; At, atrial apertnre ; Br, branchial 

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

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

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

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

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

 ty, typhlosole. 



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

 Fio-s. 18 and 19 show the relations of the branchial and 

 peribronchial cavities to one another. The peribranchial cavity 



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

 that when collectors are 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. 



