194 
THE VOYAGE OF H.M.S. CHALLENGEE. 
distinctly separated from the thorax, but the post-abdomen is conspicuous (PL XXVI, 
figs. 6, 7). The Ascidiozooids are not at all numerous, and some of them seem to be 
partially decayed. Possibly the colony may have been in a dying condition when it was 
found. 
The test is singularly soft considering its thickness. The test cells are rather 
large, and they are very abundant. The musculature of the mantle is much stronger 
than is usual in the Polyclinidse. The muscle fibres are of large size. The branchial 
aperture is very distinctly six-lobed (PI. XXVI. fig. 6), and the atrial aperture is provided 
with a prominent languet. 
The branchial sac is of quadrate form and very opaque. The endostyle is small and 
inconspicuous. 
The abdomen differs somewhat in size in different Ascidiozooids, Figures 6 and 7 on 
Plate XXVI. show two specimens, in one of which (fig. 6) the intestine reaches much 
further beyond the branchial sac than it does in the other (fig. 7 ). The stomach is smooth 
walled. 
The post-abdomen is usually three or four times as long as the combined thorax and 
abdomen, and about half their breadth. It is sometimes dilated at the end to form a 
bulb with a small terminal knob (PI. XXVI. fig. 7). Both ova and spermatic vesicles are 
found in the same post-abdomen, and the large vas deferens is always a conspicuous 
object (PI. XXVI. figs. 6, 7, v.d.), running in an irregular and convoluted course along 
the dorsal edge of the post-abdomen. 
Polydinum molle, n. sp. (PI. XXV. figs. 7-9). 
The Colony is an irregularly rounded mass, somewhat compressed laterally, and 
attached by the whole of the lower end. The upper surface is wide and convex, the 
lower is rather narrower and is flattened. The surface is uneven but smooth. The colour 
is dark grey. 
The length is 2 ’4 cm., the breadth is 3 ’3 cm., and the thickness 1'5 cm. 
The Aseidiozooids are of moderate size and not very numerous. They appear to be 
placed quite irregularly in the colony, and no systems are present. The body is from 
6 to 8 mm. in length and 1’5 mm. in greatest breadth, and the three regions are clearly 
distinguishable. 
The Test is soft and flexible. It is of a dull grey colour and moderately transparent. 
Large numbers of minute sand-grains are imbedded in it, especially in the outer layers. 
The test cells are abundant and of all shapes, most of them much branched ; some of 
those in the outer layer are very granular. No bladder cells are present. 
The Mantle is thick and opaque, but not very muscular. The chief muscle bands run 
longitudinally. 
