312 
THE VOYAGE OF H.M.S. CHALLENGER. 
atrial apertures of the various Ascidiozooids are placed in connection with the common 
cloacal apertures. There is, however, continuous test substance around the Ascidiozooids, 
and not merely a lower layer by which the colony is attached, and an upper layer from 
which the Ascidiozooids are suspended, as has been described in the case of some of the 
Diplosomidse. It is really an investing mass, just like that of any other Compound 
Ascidian, except that it is much softer, and is more encroached upon by the common 
cloacal cavities and their prolongations. 
The Ascidiozooids are very distinctly divided into thorax and abdomen, and these 
two regions are very much of the same size. The thorax is usually rather the longer, 
and the abdomen the broader of the two (see PL XLII. figs. 6, 7, 8, 9). The thorax 
and abdomen are connected by a very short and narrow pedicle, consisting mainly of the 
oesophagus and the rectum (PL XLII. fig. 9). The vascular ectodermal appendages given 
olF by the Ascidiozooids are generally two in number, one longer and one shorter 
(PL XLII. figs. 8, 9). They run posteriorly through the test, and terminate in short 
wide swellings or bulbs (PL XLII. fig. 10), on which the ectoderm cells become columnar 
in form (PL XLII. fig. 11, shows a terminal bulb in optical section). 
The branchial sac is longer than it is wide (PL XLII. fig. 8, br.s.). It has in most 
Ascidiozooids four rows of stigmata, and there are on an average ten stigmata in each row. 
The musculature of the branchial sac is well developed (PL XLII. fig. 14, m.f.). A 
strong band runs along each transverse vessel, and gives off fibres which pass upwards 
and downwards into the interstigma tic vessels. As the stigmata in adjacent rows usually 
alternate with each other, the same muscle fibres do not pass directly upwards from 
an interstigmatic vessel to one above, but always interlace with the muscle fibres in the 
transverse band, before passing to the next row (see PL XLII. fig. 14). The ciliated cells 
on the sides of the stigmata are distinct. In the branchial sacs of young Ascidiozooids 
the stigmata are much smaller, and are relatively shorter (PL XLII. fig. 15), w’hile the 
transverse vessels are wider. Two Copepoda were found in the branchial sac of one of 
the Ascidiozooids examined. 
The endostyle is of moderate size ; its course is straight (PL XLII. fig. 8, en.). The 
dorsal languets are long and narrow, and are nearly tentacular in form. Their bases 
equal in breadth the space between the two dorsal muscle bands in the mantle 
(PL XLII. fig. 16, d.L). In the young Ascidiozooid examined (see PL XLII. fig. 15, 1.) 
the languets are relatively much shorter and stouter. They evidently become elongated 
as they grow older. 
The tentacles (PL XLII. fig. 13, tn., tn'.) are large and regular. The longer ones are 
as a rule about twice the size of the intermediate shorter tentacles. In some cases the 
dorsal, the ventral, and the two median lateral tentacles are rather larger than the four 
remaining long ones, thus producing three orders — four primary, four secondary, and 
eight tertiary — alternately placed. 
