156 
THE VOYAGE OF H.M.S. CHALLENGER. 
internal longitudinal bars in the branchial sac. No calcareous spicules like those of 
Culeolus are present in the vessels of Pharyngodictyon, but a few rather large muscle 
fibres are found traversing them. The walls of the vessels are covered with squamous 
epithelium except on the internal edges of the longitudinal bars, where the cells become 
columnar and have tapering pointed free ends (PI. XXI. fig. 13). 
The endostyle is large and conspicuous. In side view (PI. XXI. fig. 5) its course is 
nearly straight, but when seen in front view (PI. XXL fig. 16) the edges are found to be 
very much undulated. The very considerable breadth of the organ is also seen in this 
view. 
The tentacles are large and fairly numerous. They are wide at their bases and recall 
by their appearance (PI. XXL fig. 14) the tentacles of some species of Styela amongst 
the Simple Ascidians. The upper surface of the tentacle is covered by a broad band of 
cubical epithelium (PI. XXL figs. 14, 15), while the sides and lower surface are formed 
of squamous cells. The usual septum of connective tissue is present in the interior 
(PI. XXL fig. 15). 
The alimentary canal forms a large mass (PI. XXI. fig. 5), including the widest 
portion of the body ; it is very opaque. The stomach is a large ovate, thick-walled 
organ on the dorsal edge. It has ridges projecting into the interior. The intestine 
extends posteriorly for a short distance and then turns sharply to the ventral side and 
anteriorly. It is a large tube irregularly swollen with faeces at intervals. It forms the 
ventral edge of the visceral mass, and then crosses the oesophagus to reach the dorsal side 
of the branchial sac (PL XXL fig. 5). In sections the stomach and intestine are found 
to contain great masses of a soft greyish colour composed mainly of Diatoms.^ 
The mature Ascidiozooids are hermaphrodite. The large genital organs he 
completely posterior to the alimentary canal in a long diverticulum of the mantle, 
which is the post-abdomen. The anterior portion of the post-abdomen (PI. XXL fig. 5) 
is narrow and contains the genital ducts. Further back it swells into a fusiform body, 
in which the ova and the spermatic vesicles lie, and behind this it is continued into a 
long narrow tail which usually terminates in a slightly swollen extremity (PL XXL 
fig. 5). One or two large yellow ova usually occupy the centre of the fusiform genital 
mass, while in front and behind are placed the small ovate or globular spermatic vesicles. 
The latter are usually about twelve or fifteen in number, and they stain a very deep red 
with picrocarmine. Each has a fine duct which runs upwards to join the vas deferens 
(PL XXL fig. 17). This tube is large and conspicuous. It is formed at the upper end 
of the genital mass by the junction of the ducts from the spermatic vesicles (PL XXL 
fig. 5), and runs forwards over the alimentary viscera to reach the rectum, up which it 
courses to its termination in the dorsal part of the peribranchial cavity. The upper 
part of the vas deferens is usually a little wider than the rest. The ova are of very 
1 The deposit on the bottom at Station 147, where this species was obtained, is Diatom ooze. 
