330 
THE VOYAGE OF H.M.S. CHALLENGER. 
projection from one edge of the disc (PL XLIII. fig. 1). 
two colonies are as follows 
Length, ...... 
Breadth, ...... 
Thickness, ...... 
Length of peduncle, ..... 
Average thickness of peduncle. 
The exact dimensions of the 
A. B. 
6-5 
cm. 
7-0 
cm. 
7-3 
)) 
8 ’5 
j? 
3 0 
jj 
2-0 
>> 
2-0 
jj 
3-0 
1-0 
1-5 
The thickness gradually diminishes from the point where the stalk is attached to the 
opposite end, which was probably the upper edge of the colony. The peduncle rapidly 
tapers downwards from the point where it joins the disc to a small irregularly shaped 
area of attachment (PI. XLIII. fig. 1). The colour is on the whole a pale slate-grey. 
The anterior ends of the Ascidiozooids have more of a dull bluish tint, and the investing 
mass between them is whiter. The peduncle is of an earthy brown colour, and is 
strongly wrinkled transversely (PL XLIII. fig. 1). 
The Ascidiozooids are conspicuous externally over the greater part of the surface. 
The rounded areas visible vary from 1 to 6 mm. in length ; the general size is 4 mm. 
As a rule they are larger at the lower edge of the colony near the peduncle, and smaller 
at the opposite end (PL XLIII. fig. 1). This is exactly what would be expected if the 
young Ascidiozooids were formed at the upper free end of the colony, and is the reverse 
of the condition found in Colella, amongst the Distomidae, where the new Ascidiozooids 
are added to the base of the colony. 
In some places the apertures are distinctly four-lobed, in others they are circular, and 
sometimes they appear irregular ; when they are widely open they are always circular in 
outline. In an Ascidiozooid which appears on the surface as 4 mm. in diameter, the 
apertures are about 2 mm. apart (PL XLIII. fig. 1). 
A section through the colony (PL XLIII. fig. 2) shows that the Ascidiozooids occupy 
only an outer zone which takes up about one-third of the thickness, leaving a central 
region formed of test only. The Ascidiozooids are of rounded or ellipsoidal form, and 
vary consideraljly in antero-posterior length ; all sizes from 4 to 8 mm. are common 
(PL XLIII. fig. 2). The body is nearly equally wide throughout its length, and there is 
no division into thorax and abdomen. 
The test is solid and massive ; in the interior of the colony, although firm, it is not 
at all hard ; it lias an elastic feeling, and is moderately tough. The minute fibrillation 
extends through the test matrix in all directions. The test cells are small and not 
numerous. They are mostly of rounded forms. The vessels are a conspicuous feature 
in sections of the test. They are mostly of small calibre, but they branch frequently 
(PL XLIII. fig. 3) and their terminal swollen bulbs are large. 
The musculature of the mantle is moderately strong. The numerous fine muscle 
bands form a close network (PL XLIV. fig. 9, m.). The sphincters are well developed. 
