IIG 



MORPHOLOGY. 



seen extending themselves in the form of long processes and threads of sarcode, sometimes simple 

 and undivided, sometimes breaking themselves up into branches, sometimes stretching themselves 



out as free processes into the surrounding water, and 

 sometimes seeming to flow over the surface of the 

 hydrosoma in simple or branching streams ; and then 

 again the whole will slowly withdraw itself into its 

 chitinous receptacles, leaving not a trace visible of 

 those wonderfully extensile processes and filaments of 

 sarcode into which it had just before transformed itself. 

 In all this the clusters of thread-cells, when they exist, 

 remain quite stationary, being never carried out with the 

 sarcode in its pseudopodial prolongations.' 



Thread-cells. — The most characteristic elements of 

 the ectoderm are the thread-cells. They occur under 

 various forms throughout the whole of the Ccelenferata, 

 and though analogous bodies are occasionally found in 

 some other invertebrate groups, they are nowhere 

 so abundant and cliaracteristic as in the Ccelenteratar 



The form of the thread-ceil varies in different 

 species of hydroids, and even in different parts of the 

 same animal ; but it consists essentially of a containing 

 capsule, and a contained filament, which admits under 

 certain conditions of being projected from the capsule 

 (woodcut, fig. 52). The investigation of the thread- 

 cell, with the view of obtaining a knowledge of its struc- 

 ture and mode of action, is one of the most difficult 

 tasks in the anatomy of the Hydroida. The minute 

 size, great transparency, and almost entire xmiformity of 

 action on the light, of all the parts of these really com- 

 plex bodies, and the rapidity with which their charac- 

 teristic function is performed, renders their study one 

 which requires no ordinary patience and practice in 

 microscopical observation, and, notwithstanding the labour 

 which has been bestowed upon them, our knowledge of 

 the thread-cell is by no means in all points satis- 

 factory. 



The large thread-cells which occur in the tentacles 

 and body of Hydra, or in the capitula which terminate 

 the tentacles in Coryne, may be taken as presenting the 

 most usual type of these bodies among the Hydroida. 



' See my " Report on the Reproductive System of the Hydroida," in ' Brit. As. Rep.' for 1863, 

 and a paper " Ou the Occm-rence of Amocbiform Protoplasm and the Emission of I'seudopodia in the 

 Hydroida," 'Ann. Nat. Hist.' for March, 18Gi. 



"' Among the more recent authors who have studied the thread-cell in the Coslenterala, refe- 



Portion of a ramulus of Antennularia antennina, 

 with hydrauths and nematophores. 



a, Hydranth extended ; b, bydraath retracted ; 

 e, hydrotheca; (/, d, d,d, consecutive segments of the 

 ramulus ; e, e, azygous or mesial nematophores, with 

 tiieir sarcode contents quiescent ; e', e', e', azygous 

 nematophores, with the sarcode contents emitting 

 pseudopodial prolongations; f, gemminate or lateral 

 nematophore-s with pseudopodial prolongations of the 

 sarcode. 



