246 GILBERT C. BOURNE. 



of bands whicli run back down the length of the arm and 

 are inserted^ like the dilators of the branchial orifice, on the 

 ring of thickened tissue surrounding the mouth. One of 

 these longitudinal bands, situated on the internal or adoral 

 aspect of the arm, is especially large and thick (fig, 5, mus. 

 flex.) and is to be regarded as the flexor muscle of tlie arm, 

 bending it inwards towards the branchial orifice; while the 

 slips derived from the outermost band of the sphincter would 

 appear to act as extensors of the arm, lying as they do on its 

 outer or aboral aspect. One may infer, from the arrange- 

 ment of the muscles, that the arms in the living animal 

 could be closed rapidly and powerfully over the branchial 

 orifice, and would therefore be serviceable in capturing prey; 

 but it should be noticed that the lateral offsets of the 

 muscular core of each arm, which obviously fit into the 

 pinnae, contain no muscular slips. It must be concluded, 

 therefore, that while the arm as a whole is movable, the 

 pinnae are incapable of independent movement. 



The presence of muscular arms surrounding the branchial 

 opening, though not unknown, is an unusual feature in 

 Tunicata. The only Tunicate having arms comparable in 

 size and complexity to those of Oligotrema is Octacuemus, 

 and there is some resemblance in the arrangement of the 

 muscles in these two forms. But the anatomy of Octacnemus 

 differs so much in other respects from that of Oligotrema 

 that there can be no question of close relationship, and one 

 must look for a nearer homology to the six denticulate 

 muscular lobes surroiinding the branchial aperture of 

 Cteniculla, which in turn are modifications of the six simj)le 

 lobes characteristic of the branchial aperture of the Molgu- 

 lida). Other Ascidians, of course, have similar lobes, but the 

 number is not the same. The circumoral lobes of Ctenicella 

 are described by de Lacaze Duthiers as muscular, and as 

 having each one terminal and two pairs of lateral pinna:. If 

 they were largely increased in size, and their lateral })inniO 

 were multi[)lied and complicated by branching, and the circlet 

 of lobes were somewhat further removed from the branchial 



