146 
Proceedings of the Royal Soeiety 
distinct, more or less triangular peritubercular area, whicli is a diver- 
ticulum of the prebran chial zone, and is formed by the dorsal ends 
of the peripharyngeal band bending posteriorly to meet the anterior 
extremity of the dorsal lamina. 
The dorsal tubercle is, in the simplest form known, a funnel- 
shaped de]3ression having its wider, circular, anterior end opening 
freely into the branchial siphon (the tube leading from the branchial 
aperture to the branchial sac), and separated off from the prebranchial 
zone by a raised edge or lip. The opposite narrower end is con- 
tinued into a fine canal running dorsally or posteriorly in the 
direction of the nerve ganglion. This simple condition is found 
in Molgula pedunculata, ; in Fjugyra kerguelenensis, the aperture is 
still wide, although its edge is square in place of being circular. 
In other simple and compound Ascidians, the anterior half of 
the edge has been apparently pushed backwards so as to become 
invaginated and closely applied to the posterior half, thus 
reducing the circular aperture to a crescentic or semicircular slit. 
This condition is found in Corella parallelogramma. In most 
other forms, more or less complication is produced by the ends of 
the slit, or “ horns ” as they may be called, being prolonged often 
to a very great extent, and coiled in various directions, sometimes 
producing beautifully regular and closely placed spirals, as in Molgula 
gigantea. The patterns produced by this curving of the horns are 
very numerous, and often complicated ; but their value in classifica- 
tion is slight, since they differ sometimes to a considerable extent in 
different individuals of the same species, and on the other hand, are 
sometimes very similar in members of different genera or even families. 
Some few forms are known in which, in place of being a single 
curved or spiral organ, it is formed of several distinct irregularly- 
shaped apertures, as in Cynthia irregularis ; this has evidently been 
formed by the lips of the primitive simple dorsal tubercle having 
become so folded as to divide the single aperture into several. 
Lastly, there are cases in which the organ is even more complicated, 
as in Boltenia ’pachydermatina, so that it becomes very difficult to 
trace its derivation from a simple circular opening. 
This variously shaped organ is histologically merely a depression 
in the connective tissue of the mantle, lined by epithelium con- 
tinuous with the squamous epithelium covering the prebranchial 
zone. These cells are modified upon the edges of the slit into first 
