34 
STRUCTURE AND PHYSIOLOGY 
for the present, to consider all the ascidian as compound poly- 
pes.* There is nevertheless a remarkable difference between 
them and the hydraform tribes in their mode of composition. 
In the latter the polypes are simply developements of the com- 
mon central fleshy mass, identical with it in structure and tex- 
ture ; in the former each individual is a distinct organism, and 
the medium which binds them together, whether vascular or li- 
gamentous, 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 percur- 
rent thread. To express this distinction we shall call the hydra- 
form compound polypes, and the ascidian aggregated polypes. 
The body of the ascidian polypes is lengthened, somewhat 
cylindrical or at times bulged at the base, and when at rest lies, 
in the form of a syphon, doubled up upon it- 
self in the cell, (Fig. 2, f) to which it is con- 
nected 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 surrounded by a single 
row of tentacula, (Fig. 3, a,) which are solid, 
filiform and non-contractile, for the animal 
can only shorten them, excepting to a 
slight extent, by rolling them up in a spiral 
manner : they are apparently smooth, but with a high mag- 
nifier it is ascertained that they are clothed with numerous 
fine cilia, j which are in ceaseless motion, and are supposed 
* “ 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 Flustrse is incompatible with their existence 
as separate and independent beings, but is quite analogous to what we are ac- 
customed to observe in Cellarias, Sertulariae, Plumularise, and many other well- 
known compound animals.” — Grant in Edin. New Phil. Journ. iii. 116. See 
also Blainville, Man. d’Actinologie, p. 99. 
| The figure, for which I am indebted to my friend Mr William Scott, repre- 
sents the polypes of Flustra membranacea in a state of retraction. 
| For a history of this discovery, written with great learning and impartiality, 
Fig. 2. 
