332 M. Ussow’s Zoologico-Embryological Investigations. 
Among the subsidiary organs of the alimentary apparatus 
I reckon also an enigmatical glandular organ, detected by me 
in some simple (Ascidia intestinalis, canina, Cynthia micro- 
cosmus) and social (Clavellina lepadiformis) Ascidia. ‘This 
sac-like gland, consisting of two coalescent portions, is situated 
sometimes beneath*, sometimes abovet the central ganglion. 
It consists of numerous, very variously bent, czecal tubes, lying 
within a common envelope. ‘Their cavities, which are lined 
with simple cylinder-epithelium, contain spherical bodies of 
various sizes. All the tubes of each half of the gland unite in 
its centre to form a more or less thick tube, or an efferent duct, 
which opens into one of the nearest cavities of the ciliated pit. 
This gland occurs very early in young Ascidia; but I have 
not succeeded in ascertaining from which germ-lamella, or 
from the parts of which organ, it is formed (by eversion ?). 
The Tunicata are not Mollusca. Even without taking into 
consideration the mode of embryonal development, a com- 
parison of the plan of structure of the different Mollusca with 
that of the Tunicata suffices to refer the latter with more. 
propriety to the Vermes. The simple cardiac tube, the absence 
of the cesophageal ganglia and their commissure, the complete 
absence of the foot, the curvatures of the intestinal canal 
directed towards the heart, the existence of the outer mantle, 
and the peculiarities of its structure, mode of formation, and 
chemical constitution, the variability in the directions of the 
contractions of the cardiac tube, &c., all draw a more or less 
sharp boundary-line between the Tunicata and the Mollusca. 
The Tunicata approach most closely to the Bryozoaf. 
On the other hand, it must be admitted that the simplicity 
in the structure of the nervous system (the Appendicularie 
excepted) and the cardiac tube, the relation of the respiratory 
organ to the upper part of the intestinal canal (Balanoglossus), 
the indistinct separation of the inner mantle from the muscular 
layer (dermo-muscular layer), and the very general alternation 
of generations, constitute characters by which the Tunicata in 
some degree approach the type of Vermes§ (to which, thanks 
* In Ascidia intestinalis and canina and Clavellina lepadiformis. 
+ In Cynthia microcosmus. 
{ Huxley, Lect. on Comp. Anat. pp. 80, 85; Hiickel, Gen. Morphol. 
Bd. ii. p. citi; Allman, Brit. Assoc. Rep. 1850, and Trans. Roy. Irish 
Acad. vol. xx. 1852, p. 275; Van Beneden, Mém. Acad. Belg. vol. xx. 
pp. 54-58. 
§ Gegenbaur, Vergl. Anat. 2te Aufl. p. 128; Hiickel, Naturl. Schdp- 
fungsgesch, 4te Aufl. pp. 448, 466, 467, 
