Natural History of British Zoophytes. 
233 
axis, and are hence truly compound beings ; and since the Vesiculi- 
fera, which are admitted to be composites, belong unquestionably to 
this remarkable form of animated entities, it is safer, for the present, 
to consider all the ascidian as compound polypes.* There is never- 
theless a remarkable difference between them and the hydraform 
tribes in their mode of composition. In the latter the polypes are 
simply developements of the common central fleshy mass, identical 
with it in structure and texture ; in the former each individual is 
a distinct organism, and the medium which binds them together, 
whether vascular orligamentous, has its own peculiar character. The 
one we may compare to a chain of which all the links are welded, 
the other to a necklace where the beads are strung together by a 
percurrent thread. To express this distinction we shall call the 
hydraform compound polypes, and the ascidian aggregated polypes. 
The body of the ascidian polypes is lengthened, somewhat cylin- 
drical or at times bulged at the base, and when at rest lies, in the 
form of a syphon, doubled up upon itself in the Fig. 1. 
cell, (Fig. 1, t) to which it is connected by a 
tendon at the bottom, and by the duplicature of 
a thin membrane round the aperture, so that it 
is impossible it should ever voluntarily leave the 
cell to swim at large, as Baster and others have 
maintained. The head or upper end is surround- 
ed by a single row of tentacula, (Fig. 2, ,) which 
are solid, filiform, and non-contractile, for the 
animal can only shorten them by' rolling them up 
in a spiral manner : they are apparently smooth, 
but with a high magnifier it is ascertained that 
they are clothed with numerous fine cilia, J which are in ceaseless 
* " The polypi are most intimately and inseparably connected with the axis 
by three parts of their body, and are only digestive sacs or mouths developed by 
the axis, as in all other zoophytes, for the nourishment of the general mass. By 
the axis of a zoophyte, I understand every part of the body excepting the polypi, 
whether of a calcareous, horny, or fleshy nature. The exact mathematical ar- 
rangement and forms of the cells of Flustrae is incompatible with their existence, 
as separate and independent beings, but is quite analogous to what we are ac- 
customed to observe in Cellaris, Sertularise, Plumulariae, and many other well- 
known compound animals." Grant in Edin. New Phil. Journ. iii. 116. See 
also Blainville, Man. d'Actinologie, p. 99. 
f The figure represents the polypes of Flustra membranacea in a state of re- 
traction. 
f The Rev. Dr Fleming is the first who discovered this structure and its im- 
portant use in creating respiratory currents. See Wern. Mem. iv. 488-9. The 
