ZOOPHYTA ASCIDIOIDA. 
239 
tween the polypidoms there is an apparent affinity. The Crisiadge 
are not unlike the Sertulariadse, and it is still disputed whether 
some Gemmicellarise appertain to this family or to the Flustra ; 
the resemblance between the Sertularise and the Vesiculariadae 
misled even Lamarck to their union under one genus ; and their 
names would seem to imply that the framers of the genera Al- 
cyonidium and Alcyonella believed them to be in a family re- 
lationship to Alcyonium. These are examples which prove the 
fallacy of outward characters ; and how darkly the naturalist 
must grope his way who would walk through Nature’s labyrinth 
without the ariadnaean thread that the anatomist alone can give 
him ! In the instance before us he has demonstrated that the 
resemblances indicated above imply no propinquity in their ob- 
jects. The ascidian polypes the Creator has cast in the mould 
— not of the radiata — but of the mollusca, — yet with such a con- 
siderable variation as to mark their ordinal distinctness ; for the 
Mollusca Tunicata — their nearest allies — are not protrusive from 
their cells as these polypes are ; and this seeming slight dis- 
crepancy connects itself, perhaps of necessity, with a total change 
in the position and nature of their respiratory organs. Interior 
and immotive in the one tribe, they line, in a reticular pattern, the 
parietes of a sac capacious enough to contain a sufficiency of the 
aerating fluid ; while in the other they clothe the exsertile ten- 
tacula in the form of cilia which must be placed outwards amid 
the circumfluent waters before they can play and fulfil their func- 
tions. 
Notwithstanding the great diversity in the forms of the poly- 
pidoms of this order, there appears to be a very remarkable uni- 
formity in the habit and structure of the polypes. The body lies 
doubled up in the cell (Fig. 39, a, b.), its oral extremity crowned 
with a circle of long filiform ciliated tentacula.* From the 
centre of this circle the mouth opens into a sort of pharynx 
(Fig. 39, a, 1.) which begins the oesophageal tube, generally of 
* At page 34 the tentacula of the Ascidioida are stated to be solid : an error 
which Dr Farre enables us to correct. In Alcyonidium, “ with an amplifying 
power of 200 linear,” “ they are seen to be tubular throughout, and to have an 
aperture at each extremity; that at the base apparently communicating with a 
fine canal round the oral rim, which probably unites the tentacular canals into 
one circle.” Phil. Trans, an. 1837, p. 406. This is a structure very like that 
in the two preceding orders. 
