T o the naked eye it appears as a.white moveable point; but
when examined by the microfeope, a tail, projecting from the
lower part, is difcovered,- and a double rotatory inftrument is feen,
which it can conceal or difcover at pleafure. It has been feen
and deferibed by moll microfcopical writers ; but as Mr. Baker’s
feems to us to be the mod perfect,' we lhall principally follow his
account o f it. •
He difcovered three fpecies o f them, two o f which are included
under the vorticella urceolaris. Fig. 33, 34, 35, are of the firft
fpecies ; Fig. 36, 37, 38, are o f the fecond- kind. The firft fort,
when extended, is about twice as long as it is broad. It is contained
in a fhelI ; the fore-part o f this is armed with four {harp
teeth, or points ; the oppofite fide has no teeth, but is waved, or
bent, in two places, like the form o f a Turkilh bow. - At the
bottom there is a hole, through which it pulhes the tail. It
fattens itfelf by this tail to any convenient fubftance, when it intends
to ufe it’s rotatory organs; but when it is floating in the
water, and at all other times when not adhering to any body, it
wags the tail backwards and forwards fomething like a dog.
We may confider it as divided into a head, thorax, and abdomen
; each o f which may be extended and contracted confider-
ably: it can by dilating all three protrude the head beyond the
’fhell, or by contracting them, draw the whole body within the
fame.
The head when extended divides itfelf into two branches, between
which another part, a kind o f probofcis, is pufhed out;
at the end o f this are two fibrils.,, that appear when they .are at
reft
M l C R O S C - O P I C A L E S S A Y S . 4 47
reft like a broad point, but which can be moved to and from
each other very brilkly with a vibratory motion, fee Fig. 33.
The form and fituation of the two branches is fometimes
changed, the ends thereof becoming more round, and the vibratory
motion is altered to a rotatory one: this alteration is repre-
fented at Fig. 34: the head al'fc appears in this figure. The
thorax, is annexed to the lower part o f the head, it is mufcular’;.
within it there is a moving inteftine, which has been fuppofed to
be either the lungs or the heart of the little creature, fee b, Fig..
33 and 34-
A communication is formed between the thorax and the abdomen
by means of a fhort veffel c, whofe alternate Contractions.
and dilatations occafion the abdomen to rife and fall alternately,,
having at the fame time a fort o f periftaltic motion. The food
is conveyed through this veffel into the abdomen, where it is
digefted ; ’ it is then difeharged by the anus, which is placed near
the tail;
The tail lias three joints, and'is cleft or divided at the extremity,.
by which means it can better, fatten itfelf to fuitable objects,.
It is in general, projected from the lower end of the fhell, moving
nimbly to and fro, ferving the animal as a rudder when it is.
fwimming to direCl it’s cou-rfe..
When the water in which the little animal is placed is nearly
dried away, or when it [has a mind -to compofe itfelf to reft, it
contrafts the head and fore-part of the body, and brings them
down, into the (hell, and pulls the tail upwards, fo that the. whole
of