$$ 27, 28. THE POLYPI. 39 
Anthozoa have the skin, and especially the tentacles, covered with cilia of 
this nature,” these last cannot be regarded as forming a differential charac- 
teristic between them and the Bryozoa, as has been done by Ehrenberg. 
» § 27. 
The skin of many polyps is quite remarkable in having nettling or 
poisonous organs, to which it is only of late that the attention has been 
“directed. They consist of transparent vesicles, having a dense membrane, 
of a round, oval, or cylindrical form, containing a clear liquid, and a very 
delicate filament of variable length, which is usually spirally coiled. By 
the least irritation of the skin, the filament is thrown out of the vesicle, of 
which it appears to be only a prolongation. These filaments adhere to 
objects coming in contact with the skin, and in this way the vesicles in 
question are separated from it.? These organs are probably the cause of 
the nettling sensation felt when certain polyps are handled. 
§ 28. 
Still more interesting are organs analogous to those just mentioned, and 
which belong to various species of Hydra.” They are found not only on 
the arms, but also upon the skin of the body and foot. They consist of 
oval vesicles, having a very long and delicate filament, which is slightly 
swollen and viscous at its free extremity, while the opposite one is directly 
continuous with the conical neck of the vesicle. The neck of each vesicle 
is surrounded by three hooks curved backwards. These are always elevated 
when the skin of the animal is irritated, and especially that of the arms 
when they seize their prey. This last is then wound about by the free, 
viscous end of the filament, and the attached vesicle being torn from the 
body, the whole is often entangled in the arms of adjacent polyps. When 
this occurs, the vesicles hang by their hooks to the arms of the polyps; and 
it is this that has given Ehrenberg the opinion that the vesicles are detached 
by their round extremity, that these animals watch their prey with the 
hooks erected, and that the vesicles and filaments can return into the inte- 
rior of the arms.® But it is probable that they (the hooks) act more as 
poisonous than as prehensile organs ; for if those from the arm of a Hydra 
seize upon a Nats, a Daphnia, or alarva of Chironomus, these last quickly 
die, even if they escape immediately after being taken. * 
1 Erdl has seen very distinct ciliated epithelium 
in <Actinia and Veretillum. (See Miiller’s 
Arch, 1841, p. 423.) 
2 Abhaudl. d. Berl. Akad. 1884, p. 255, 377. 
1 These nettling organs, which are much more 
common in the lower orders of the animal kingdom 
than was at first supposed, are yet quite imper- 
fectly known. .Wagner first discovered them in 
the Actinia, although he regarded them at first 
as the spermatic particles of these animals. (Wieg- 
mann’s Arch. 1835, II. p. 215, Taf ITI. fig. 7, also 
7841, I. p. 41; Icones Zoot. Tab. XXXIV. fig. 24.) 
hese researches have been extended by LE£rdi, 
who has shown that they also exist with Veretillum 
and Alcyonium. (Miuiller’s Arch. 1841, p. 423, 
Taf. XV. fig. 3--6 and 8,9.) In Alcyonium, Erdi 
has observed the filament take, on its departure 
from the vesicle, first a riband-like, and then a 
spiral aspect. In Desmophyllum  stellaria 
(Ehrenberg), I have seen these cylindrical organs 
having a long spiral filament. With Edwardsia, 
Quatrefages has found these organs upon the 
whole surface of the body, as well as upon the 
arms. (Ann. d. Sc. Nat., Zool. 1842, XVIII. p. 81, 
Pl. II. fig. 4-6.) For the nettling organs of the 
Tubulariae and the Actiniae,see also Wagner in 
Muller's Arch. 1847, p. 195, Taf. VIII. 
1 These were first described by Ehrenberg. 
(Mittheil. a. d. Verhandl. d. Gesellschaft naturf. 
Freunde zu Berlin 2 tes. Quartal, 1836, p. 28 ; also, 
Abhandl. d. Berl. Akad. 1835, p. 147; 1836, p. 
138, Taf. II.) They have been carefully studied 
by Erdl (Miiller’s Arch. 1841, p. 429. Taf. XV. 
fig. 10--13). 
2 Ehrenberg has figured, ideally (Abhandl. d. 
Berl. Akad. 1836, p. 133, Taf. II. fig. 1) an Hydra 
in the act of seizing its prey with extended hooks. 
In reality this animal is never thus seen. 
