114 THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

 ous production. 



All the Hydrozoa are provided with tentaculcr 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 Coelenterata. In its most perfect 

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

 the 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 sac at its thicker or basal end, while its 

 other pointed end is free. Very slight pressure causes the 



Fig. 14.— Sacculus of a tentacle with nematocysts of Athorybia: J., peduncle or 

 stalk, and B, involucrum of the sacculus 0; J), filaments ; d, ectoderm ;e, endo- 

 dertn ; /, nematocysts; 1, small nematocysts of the filaments and involucrum; 

 2, 3, larger nematocysts of the sac ; 4, largest nematocysts. 



thread to be swiftly protruded, apparently by a process of 

 evagination, and the nematocyst now appears as an empty 



Hydrozoon travels for a certain distance alon<r the same great highway of de- 

 velopment as the higher animal, before it turns off to follow the road which 

 leads to its special destination." 



In this passage of my work on the " Oceanic Hydrozoa " (1859), I expanded 

 the idea enunciated in the memoir on the Medusae here referred to, that " the 

 outer and inner membranes appear to hear the same physiological relation to 

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

 (Fig. 13), exhibiting the relations of the different groups of the Hydrozoa, was 

 published in the Medical Tunes and Gazette in June, 1856. 



