20 
THE VOYAGE OF H.M.S. CHALLENGER. 
(PI. XL fig. 10), but they may, on the other hand, he tapering tentacle-like processes 
{e.g., Atopogaster aurantiaca, PI. XXIII. figs. 11, 12) with no antero-posterior flattening. 
They are always richly ciliated along the edges. 
The Tentacles. 
The tentacles are simple in all Compound Ascidians, and are usually few in number 
and placed at regular intervals round the base of the branchial siphon. Eight and 
sixteen are usual numbers, but occasionally only four or even two [Polycyclus cyaneus, 
V. Drasche) are present. In other cases they are more numerous, and in Tylohranchion 
speciosum the number is large and the arrangement indefinite, as in a Simple 
Ascidian. Very often the tentacles are of two sizes, which are then placed alternately. 
Sometimes the tentacles are richly pigmented. 
The Nervous System and other Neighhouring Organs. 
The nerve ganglion is usually elliptical in form, and occupies the same position a.s in 
the Ascidise Simplices : it gives off nerves anteriorly and posteriorly. The mantle in the 
neighbourhood of the nerve ganglion is frequently pigmented, and in some Compound 
Ascidians small pigment-spots, which are probably lowly developed sense organs, are 
found along the edge of the branchial aperture just as in the case of most Simple 
Ascidians. A second small ganglionic enlargement is said to have been found on a nerve 
going to the atrial aperture in some species where the edge of the common cloacal 
aperture is well developed and sensitive. 
In close relation to the nerve ganglion, and usually on its posterior side, is placed the 
neural gland, which is comparatively slightly developed in the Compound Ascidians, and 
gives off the neural duct running anteriorly and ventrally to open into the anterior end 
of the branchial sac by the aperture of the dorsal tubercle (see -fig. 3, p. 16, gl.n). 
This opening is usually a simple circular or ovate slit (PI. II. figs. 8, 9) placed in the 
dorsal line of the prebranchial zone behind the tentacles, and having slightly raised lips 
which form the inconspicuous “tubercle.” The region of the neural duct nearest to 
the opening is generally enlarged to form an infundibulum with ciliated walls (see 
PL III. fig. 8,m/). 
The peripharyngeal bands surrounding the branchial sac at the level of the anterior 
ends of the endostyle and the dorsal lamina, and defining the posterior border of the 
prebranchial zone, in most cases bend posteriorly in the dorsal median line to bound a 
more or less triangular diverticulum from the prebranchial zone. This diverticulum corre- 
sponds to the peritubercular area in the Ascidise Simplices, but in the Ascidiae Composite 
it rarely encloses the dorsal tubercle, which is placed considerably further forward (PI. III. 
fig. 16). 
