<§>§ 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 EhrenbergS^^ 



§ 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 arras 

 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 Nais, a Dajjhnia, or a larva of Chironomus, these last quickly 

 die, even if they escape immediately after being taken. 



1 KtM has seen very distinct ciliated epithelium (Ehrenberg), I have seen these cylindrical organs 



in Actinia and f^eretillum. (See Miiller's having a long spiral filament. With Ed wardsia, 



Arch. 1841, p. 423.) Quatrefaa:es has found these organs upon the 



'■i Abhandl. d. Berl. Akad. 1834, p. 255, 377. whole surface of the body, as well as upon the 



1 These nettling organs, which are much more arms. (Ann. d. Sc. Nat., Zool. 1842, XVIII. p. 81, 



common in the lower orders of the animal kingdom PI. II. fig. 4-6.) For the nettling organs of the 



than was at first supposed, are yet quite imper- Tubvlariae andthe Actiniae, see a]so Wagner in 



fectly known. Wagner first discovered them in Mül/er''s Arch. 1847, p. 195, Taf. VIII. 



the Actinia, although he regarded them at first 1 These were first described by Ehrenberg. 



as the spermatic particles of these animals. {Wieg- (Mittheil. a. d. Verhandl. d. Gesellschaft naturf. 



mann''! Arch. 1835, II. p. 215, Taf III. fig. 7, also Freunde zu Berlin 2 tes. Quartal, 1836, p. 28 ; also, 



■■841, I. p. 41 ; IconesZoot. Tab. XXXIV. fig. 24.) Abhandl. d. Berl. Akad. 1835, p. 147 ; 1836, p. 



These researches have been extended by Erdl, 133, Taf. II.) They have been carefully studied 



whohasshownthat they also exist with Fcre?j7/«TO by Erdl {MüUer''s Arch. 1841, p. 429. Taf. XV. 



and Alcyonium. {Mailer's Arch. 1841, p. 423, fig. 10-13). 



Taf. XV. tig. 3--6 and 8, 9.) In Alcyonium, Erdl 2 Ehrenberg has figured, ideally (Abhandl. d. 



has observed the filament take, on its departure Berl. Akad. 1836, p. 133, Taf II. fig. 1) an Hydra 



from the vesicle, first a riband-like, and then a in the act of seizing its prey with extended hooks, 



spiral aspect. In Desmophyllum stellaria In reality this animal is never thus seen. 



