104 ’ CLASS III. 
to the motion of the colony, but sway gently to and fro with the 
contractions of the stem. The Feelers are cylindrical or vermiform 
structures having much resemblance to the polyps, but without an 
external opening. In some cases they are in constant motion, 
feeling about in all directions: in others they are more.sluggish 
and are loaded with the general nutrient fluid. The latter fact 
would seem to suggest a respiratory function. Some writers have 
considered them to be receptacles for the fluid forced from the 
interior of the prehensile apparatus during its violent contractions: 
and hence the name, sometimes given to them, of fluid-holders. 
But it has been objected to this explanation, that the two sets of 
organs are often at great distances from each other, and that their 
alternate action has not been established. 
The composition of all the organs in the Siphonophors cor- 
responds to that of the other acalephs. As in these, the specific 
gravity of the mass differs little from that of sea-water. The shell 
found in some families, (velella, porpita) is the thickened and hard- 
ened wall of the air-sac1.] 
On the Nervous System of Acalephs, the observations are 
hitherto imperfect. In the MJedusw EHRENBERG observed, at the 
base of each of the eight marginal corpuscles which he takes to be 
eyes, a part which he considers to be a ganglion. Each of these 
ganglia is double, or consists of two limbs that diverge towards the 
marginal corpuscle. Besides these a row of ganglia lies near the 
tentacles at the margin of the disc; every ganglion divides into two 
twigs, each of them for one of two adjoiming tentacles: so that 
every tentacle receives two nervous twigs coming from different 
ganglia. This ring of ganglia round the margin is interrupted by 
the larger double gangha of the marginal corpuscles. Moreover, 
EHRENBERG saw four groups of ganglia lying in the cavities for 
the four genital organs and in connexion with the tentacles of these 
cavities. EHRENBERG could not detect a nervous ring round the 
mouth, the usual form of the nervous system in Radials®. Some 
writers are of opinion that it by no means follows from these obser- 
vations that the parts so described are really nerves. GRANT de- 
scribed in Cydippe pileus a nervous ring with eight ganglia, each 
1 Comp. Leuckarr Zoologische Untersuchungen, s. 3—-41; KOELLIKER in Zeit. f. 
Wissensch. Zool. tv. 8. 306—315. 
? EHRENBERG Die Akalephen &e. 8. 25, 20. 
