EEPOET ON THE TUNICATA. 
19 
The transverse vessels and fine longitudinal vessels are found in all possible conditions, 
and the stigmata may vary from small circular openings (PL XXV. fig. 14) to very long 
narrow slits with parallel sides (PL VI. fig. 10), but they are never curved. The 
stigmatic ciliated cells may also vary greatly in form (see PL VI. figs. 12, 13 ; and PL XL 
fig. 7, &c.). In some cases where the internal longitudinal bars are absent, rudiments of 
the connecting ducts in the form of papillse projecting from the transverse vessels are 
present {Tylobranchion speciosum, see PL XXII. fig. 9), but generally they are entirely 
absent except along the dorsal lamina, where they form the series of languets. 
Horizontal membranes hanging from the transverse vessels are very frequently present, 
and may be wide or narrow (PL XXV. fig. 8 and PL XV. fig. 16). The number of rows 
of stigmata varies from three {Didemnum savignii) to about twenty {Atopogaster auran- 
tiaca), and the number of stigmata in each row from four or five up to a large 
number. In the embryos of most of the Compound Ascidians, even of those species 
which have a large number of rows in the adult, there are four rows of stigmata on each 
side of the sac. 
In Pharyngodictyon mirabile (p. 154, PL XXL fig. 12) the branchial sac has the 
simple structure seen in the genera Cideolus, Fungulus, and Batliyoncus amongst Simple 
Ascidians.^ It is composed of two sets of large vessels intersecting at right angles so as 
to form large quadrangular meshes. Probably the fine longitudinal vessels are unde- 
veloped, and in that case no true stigmata are present. 
The Endostyle. 
This is generally a very conspicuous organ in the body of an Ascidiozooid, and is 
relatively larger than in Simple Ascidians. It is sometimes straight, and runs antero- 
posteriorly, but more usually its course is very undulating, turning first to the one side 
and then to the other so as to form a series of closely placed folds (PL XXII. fig. 6). It 
never extends beyond the posterior end of the dorsal edge of the branchial sac. It is 
formed of elongated columnar epithelium, richly ciliated in places. 
The Dorsal Lamina. 
This may be present in Compound Ascidians, or it may be represented by a series of 
languets which are, I believe, homologous with the connecting ducts of the branchial sac 
(see p. 161). When a dorsal lamina is present {e.g., Botrylloides tyreum^ see PL II. 
fig. 8, (i.L) it is a plain membrane with neither transverse ribs nor marginal teeth ; it 
may, however, be corrugated at its free edge, especially near the anterior end. 
Languets when present are usually triangular flaps flattened antero-posteriorly 
I Part I. of this Eeport, pp. 90, 127, and 165. 
^ Named Botrylloides purpureum on the plate, and in the description, p. 41. 
