398 DR. A. FARRE ON THE STRUCTURE OF THE CILIOBRACHIATE POLYPI. 
Thus the tentacles rising first would press up the little bundle of setae (Plate XXI. 
fig. 10. a.), that lies immediately above them. The pressure continuing, the flexible 
part of the sheath (fig. 11 . b.) would be next unfolded, and the whole would then be 
expanded by the passage of the several parts in succession through them. (fig. 12.) 
It is now that the parietal muscles come chiefly into play, and by keeping the tegu- 
ments tense during the alteration in the position and form of the body, prevent any 
collapsing of its parietes, which might entangle the operation of any of its parts, and 
which, for the want of this provision, would be likely to ensue from the pressure of the 
surrounding fluid when the animal rises from its cell ; especially as there does not 
appear to be that ready communication between the interspace then left in the cell 
and the surrounding element, by which the water might flow in to supply the vacuum 
left by the change in the form of the lower part of the body, during the protrusion of 
the animal. This circumstance I had occasion to prove by noticing the forcible in- 
dentation of the stiff horny cell itself, by the pressure of the surrounding fluid, which 
in some instances followed this act, as is represented in Lagenella repens (Plate XXIV. 
fig. 2. a.). Further, by the contraction of these muscles the body may be so much 
elongated as to carry the base of the arms to some distance above the margin of the 
cell, by which the freedom of their action is considerably increased, the stomach, 
being then lifted from the bottom of the cell, hanging suspended in the visceral cavity. 
It would appear then that the act of protrusion is effected by the combined operation 
of the parietal muscles and of the alimentary canal, which in fact forms the principal 
part of the substance of the animal, the parietes being purely membranous, and having 
little else to do than to retain the fluid in which the viscera float. 
It is interesting to compare these parietal muscles with similar parts in animals of 
another class. Having been frequently struck with the close analogy which the ge- 
neral characters of the animal under consideration presents with those of the class 
Rotifera, especially in the character of the retractor muscles, I was led to compare 
the parietal muscles also with the parts which, in Hydatina senta, for example, are 
usually considered and represented as the dorsal vessel with its lateral branches. 
(For it must be understood that the parietal muscles of which I am speaking 
have no resemblance to the circular fibres that surround the bodies of vermiform 
animals, and are intimately blended with their integument, but have a totally dif- 
ferent character, being simple short filaments, occupying a very small portion only of 
the circumference of the body, and being apparently connected with the parietes only 
by their extreme points of attachment). Having procured therefore some specimens 
of Hydatina, I was not much surprised to find that the parts in the two animals were 
identical ; the transverse lines of Hydatina being obviously parietal muscles, which 
whenever the body becomes elongated may be observed by their contraction to draw 
that side to which they are attached into longitudinal folds, and to be again elongated 
whenever the body is shortened by the contraction of the longitudinal muscles, to 
which the former are evidently the antagonists. In this case the alimentary canal 
