160 . Mr. J. Alder on the British Tunicata. 



I am now acquainted with seven British species of this genus, 

 including the Ascidia conchilega of Miiller (which is a Molyula) 

 and the following species, whose internal characters, however, 

 show a considerable departure from the type of the genus. 



Molgula arenosa, Alder & Hancock. 



Body globular, hyaline, unattached, closely covered with sand, 

 excepting generally a bright smooth area on one side. Apertures 

 nearly terminal, approximated, not much produced, conical or 

 slightly tubular, retractile, set in a small circumscribed area 

 with a raised rim, appearing like a slit when contracted. Test 

 soft, glossy, transparent, and colourless, with delicate slender 

 hairs, sometimes a little branched. Mantle very thin, soft, and 

 transparent, showing the viscera very distinctly through. Ten- 

 tacular filaments bipinnate, beautifully spotted with bright opake 

 yellow. Branchial sac with six longitudinal bands on each side, 

 between which are six rows of conical eminences formed of a 

 double spiral coil of delicate vessels meeting at the apex, and 

 giving the sac a festooned appearance (PI. VII. tigs. 3 & 4). 

 There are 'also transverse bands. Ovaries double, that on the 

 right side lying within the loop of the intestine. Diameter half 

 to three-quarters of an inch. 



This species was described in the Transactions of the Tyneside 

 Naturalists' Club (vol. i. p. 197), and is also the Molgula tubu- 

 losa of Forbes in the ' British Mollusca/ who referred it (we 

 think erroneously) to the Ascidia tubularis of Rathke in l Zoo- 

 logia Danica/ It is probable, however, that he may have in- 

 cluded more than one species under this name, as there are 

 several sand-coloured Ascidians that are with difficulty distin- 

 guished on a superficial examination. His description belongs 

 to M. arenosa, but the figure represents the tubes much longer 

 and more cylindrical than the usual form. 



Externally M. arenosa does not materially differ from the other 

 Molgula, and it has the apertures divided into the same number 

 of segments ; but the branchial sac presents very marked cha- 

 racters in the beautiful spiral cones of which it is composed, and 

 in the absence of regular folds. Besides the spiral vessels of the 

 cones, others, less conspicuous, but of equal, if not greater width, 

 pass downwards from the apex to the circumference. The whole 

 structure is extremely delicate, and its fragility renders it diffi- 

 cult to preserve it entire for examination. A further difference 

 between this and the other Molgula is observable in the position 

 of the right ovary, which is situated within, and not outside, the 

 intestinal loop. 



The Cynthia Dione of Savigny has a branchial sac of a struc- 

 ture apparently similar to this, and, were it not for its four-cleft 



