REPOET THE TUHICATA. 
3B1 
and large numbers of delicate muscle bands radiate from the two siphons (PL XLIIL 
fig. 10, m.). 
The branchial sac is thin-walled. The stigmata are short, and in some places, 
especially near the dorsal edge (see PI. XLIIL fig. 4), are reduced almost to rounded 
openings. The folds are narrow, but well marked (PI. XLIIL fig. 4, hr.f.). They are 
exactly like those of many species of the Styelinas amongst Simple Ascidians. The 
internal longitudinal bars are regular, but have no papillse. The transverse vessels vary 
somewhat in width, but the sizes are not arranged with regularity. In some places 
sKght horizontal membranes are present for short distances. They are best seen along- 
side the dorsal lamina (PI. XLIIL fig. 4, tv.). 
There is a good deal of irregularity in some of the branchial sacs. In some places 
several small stigmata occur between two adjacent transverse vessels, and occasionally 
monstrous stigmata are seen which have been formed by the junction of several ordinary 
stigmata (PI. XLIIL fig. 4). The meshes are fairly regular in size, and there are three 
rows in each of the ordinary interspaces. Between the first fold on each side and the 
dorsal lamina there are only twm rows of meshes, one of the usual size, and the other, 
next the dorsal lamina, twice the ordinary size and containing about six stigmata 
(PI. XLIIL fig. 4). Next to the endostyle a row of meshes of the usual size occurs. 
The dorsal lamina (PI. XLIIL fig. 4, d.l.) is narrow and has a smooth edge. The 
ribs on its sides are continuous with the horizontal membranes of the transverse 
vessels. 
The tentacles alternate in size wnth regularity. They are so closely placed that their 
bases touch (PL XLIIL fig. 10, tn, tin!). At the base of the atrial siphon, where the 
invaginated layer of test ends, there is a slight ridge which bears a series of small 
tentacles projecting freely into the peribranchial cavity (PL XLIIL fig. 10, at.tn.). 
These atrial tentacles are much smaller than the ordinary or branchial tentacles, and 
there are only twelve of them. The position of the atrial tentacles in relation to the 
atrial siphon corresponds exactly to the position of the branchial tentacles at the base of 
the branchial siphon, but their use at the entrance to the peribranchial cavity is not 
obvious. It has been observed in some Simple Ascidians that the current of water which 
usually flows in at the branchial aperture and out at the atrial is occasionally reversed 
for a short period, the’ atrial aperture becoming inhalent. Possibly in the present 
species this habit may have become so marked as to have favoured the development of a 
circle of atrial tentacles which would act as tactile organs waving in the current of water 
entering the animal. In one of the new Simple Ascidians {Bathyoncus mirahilis) 
obtained during the Challenger Expedition, there are two circles of small atrial tentacles 
developed.^ These resemble the atrial tentacles of the present species in size and shape. 
The nerve ganglion and neural gland together form an elongated ellipsoidal mass, 
* See this Report, Part I. (in vol. vi., 1882), p. 167, pi. xxiv. fig. 12. 
