576 SUPPLEMENT 
organs of sensation, except that of touch, which is perfect in 
the tentacula, with which the mouth is armed. All the parts 
of their tissue are capable of contraction, but without any 
possibility of our perceiving in them distinct muscular fibres. 
The hydre are, however, capable of changing place altoge- 
ther, and they do so after the manner of the geometrical ca- 
terpillars, and of some leeches. Most frequently, however, 
they remain fixed by the posterior extremity, and extend, 
more or less, their body and tentacula in the different direc- 
tions necessary for seizing their prey. They feed upon very 
small aquatic insects, such as monoculi, or naiades. They 
attract them towards themselves by almost continual move- 
ments of the tentacula, enlace them in their numerous folds, 
agglutinate them by means of some secretion, or some mode 
of suction, and finally direct them towards the aperture of the 
mouth, which is in the centre of the circle formed by those ten- 
tacule. ‘This mouth, which is capable of being dilated into 
a sort of calyx, communicates into the stomach, which is hol- 
lowed in the parenchyma itself, of the body of the little ani- 
mal, without any distinct parietes, except the skin at the ex- 
terior surface. Accordingly, the similitude of the external and 
internal paries is so great, that Trembley, in one of his most 
curious experiments, has proved that the little animal may be 
turned inside out, almost like the finger of a glove, and that 
digestion and absorption can be as well performed by the ex- 
ternal as the internal side. ‘This sort of stomach has no pos- 
terior orifice, and when the prey, whether digestible or not, 
has remained there for some time, it is rejected, in the latter 
case entire, and in the former such parts are expelled, as were 
not susceptible of digestion. Hence it appears, that there is 
no choice respecting the bodies which the little animal intro- 
duces into its stomach, and that the stomach alone discrimi- 
nates by its own action upon them, whether they are suitable 
