114 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



but it is a secondary and, in the lower Ilydrozoa, inconspicu- 

 ous production. 



AH the Hydrozoa are prov^ided with tentacida' These 

 are elongated and sometimes filiform organs of prehension, 

 which are generally diverticula of both ectoderm and endo- 

 derm, but may be outgrowths of only one of them. 



Thread-ceils, or nematocysts, are very generally distributed 

 through the tissues of the Ccelenterata. in its most perfect 

 form, a nematocyst is an elastic, thick-walled sac, coiled up in 

 tlie interior of which is a long filament, often serrated or pro- 

 vided with spines. The filament is hollow^ and is continuous 

 with the wall of the s;ic at its thicker or basal end, while its 

 other pointed end is free. Very slight pressure causes the 



Fig. 14— SaccuUxs of a tenticle with nematocysts of AthoryUa: JL, peduncle or 

 stalk, and S, involucruui of the sacculus C\ Z>. filaments ; </, ectoderm ; e, endo- 

 derm ; /. neinitncysts; 1, small nematocysts of the fihiments and involucrum ; 

 2, 3, larger nem.itocyscs ol the sac; 4, largest nematocysts. 



thread to be swiftly protruded, apparently by a process of 

 evagination, and the nematocyst now appears as an empty 



Ilydrozoon travels for a certain distance aloncr the same erreat hifjliway of de- 

 velopment as tlie hi£jher animal, before it turns off to follow the road which 

 leads to its special destination " 



111 this passacre of my work on the " Oceanic Hydrozoa " (ISaO), I expanded 

 the idea enunciated in the memoir on the Medus;e' here referred to, that '' the 

 outer ;ind inner memlirunes appear to hear the same physiolosrical relation to 

 one another as do the serous and mucous lavers of the germ." The diagram 

 (Fig. 13), exhil)iting the relations of the difltercnt groups of the Hydrozoa, was 

 published iu the Medical Times and Gazatti in June, 1856. 



