418 Prof. H. James-Clark on the Anatomy and 
of the digestive cavity. There are, at least, two cf these walls. 
The inner one (p!) consists of a clear, amber-coloured, homo- 
geneous, formless tissue, in which all the organs are imbedded. 
The other, or exterior wall (p), embraces the inner one like a 
film, and has more of the character of a colourless excretion 
than a true tissue. It is thickest about midway between the 
two ends of the body, and gradually thins out to an incon- 
spicuous stratum at the anterior and posterior borders. Its 
surface is beset with excessively minute, short cilia, which, 
although occasionally and with great difficulty seen to move, 
cannot be called vibratile cilia in a strict sense, but rather 
pointed roughenings which are agitated by the varied con- 
tractions and expansions of the tissue from which tbey arise. 
The thickness of this wall is more or less deeply corrugated, 
principally in a longitudinal direction (fig. 14,7,7), and to a 
certain extent independently of the irregular folds and furrows 
on the outer surface of the inner wall *. 
§ 7. Tue Crrcutatory System.—It would seem a little re- 
markable, at first thought, that the Vorticellide, which hold the 
highest rank among Infusoria, should possess a circulatory 
system which, in all but one genus, seems as simple in cha- 
racter as that of the lowest forms of the class, and apparently 
much less complicated than in Stentor and Paramecium and 
others of the leotropic division. If, however, we look upon the 
numerous contractile vesicles of Amphileptus, Trachelius, &c., as 
indications of a diffuse, lowly organized circulatory system, and 
upon the fewer branching vesicles of Paramecium, Spirostomum, 
Stentor, &c., as tendencies to a greater degree of concentration, 
then the unique contractile organ of Vorticellidans would repre- 
sent the consummation of this process, and consequently the 
most elevated status of the system as it exists in this class of 
animals. 
The contractile vesicle (cv) of Trichodina is a simple cavity 
which lies near the ventral side (figs. 8, 11) of the animal, a 
little to the left of the axial plane (figs. 12, 13), and conse- 
quently on the same side of the cesophagus, and about halfway 
between the anterior and posterior truncate ends of the bodyt. 
It contracts once in fifteen seconds. The systole occupies 
* See, for further details, the section (§ 3) on the form of the body. 
+ There is a singular error im Stein’s figure (Infusionsthiere, 1854, 
taf. vi. fig. 56) of the animal as seen from the basalend. The view of the 
base is underlain by a view of the anterior end of the body; but the 
latter is posited as of seen from the front. To correct it, the contractile 
vesicle should lie to the right of the cesophagus, and the sigmoid flexure of 
the vestibule and cesophagus should be reversed. 
