72 BRITISH TUNIC ATA. 



referred (we think erroneously) to the Ascidia tuln- 

 laris of Rathke in ' Zoologia Danica.' * It is probable, 

 however, that Forbes may have included more than 

 one species under this name, as there are several sand- 

 covered Ascidians which are with difficulty distinguished 

 on a superficial examination. His description belongs 

 to E. areiioxa, but the figure represents the tube much 

 lono-er and more cylindrical than the usual form. 



O / 



As a pretty full account is given of the internal 

 organization in the generic description, much need not 

 be said here 011 the subject. 



The mantle (PL XXIX, fig. 1), in specimens pre- 

 served in spirit, is of a purplish colour, and is so 

 delicate and transparent that the viscera are sharply 

 defined at the external surface. The tubes are rather 

 slender and conical, and the radiating muscles at their 

 base are very regularly disposed and strongly marked 

 on account of their dark purple hue ; they thicken 

 gradually towards their lower extremities which are 

 spread out like the rays of a fan, as all terminate at 

 the same transverse line ; but as they are not con- 

 spicuous in a fresh state it would seem that the colour 

 may be owing to the effect of the alcohol. The tissue 

 of the branchial sac (PI. XXIX, figs. 2 and 3) is 

 extremely delicate ; on the external surface (PL XXIX, 

 fig. 2) the primary vessels divide the spirals into trans- 

 verse rows, the spirals appearing as large, hollow, oval 

 pouches, six or seven in each row. On the inner sur- 

 face (PL XXIX, fig. 3) the spirals project boldly into 

 the cavity of the sac, each forming a square area defined 

 by the longitudinal bands and primary vessels. The 

 endostyle reaches to the bottom of the sac and has on 

 each side a rather wide, smooth space devoid of 

 secondary vessels. The longitudinal bands or rods are 

 not much arched ; they are wide, delicate, and mem- 

 branous, and abut below on the oral lamina, which 

 extends downwards almost to the bottom of the dorsal 

 margin of the sac. The mouth, being placed at the 



* [Forbes here referred to this species as "Ascidia tululosa Eathke."] 



