SENSATION. 137 



seem to be the result of tlic contractility of the sarcodc, probably combined with the general 

 elasticity of the tissues. 



I have already shown that the substance which fills the nematophores of the Plumularkla; is 

 mainly composed of a similar sarcode, which here, however, is not confined in cells; and it can, 

 therefore, extend itself beyond the surface of the hydroid in the form of long ])sendopodia, which 

 will occasionally even branch exactly as in certain JRhizopoda (sec woodcuts, figs. 50, 51). The 

 remarkable capsules tilled with thread-cclls, which are borne along the marginal tentacles of the 

 medusa of Gemmaria implexa (PI. VII, fig. 3, 4), are supported on peduncles of extraordinary 

 extensibility. In these peduncles we have also an example of true sarcode identical with that of 

 the BJdzopoda. Strethill Wright has also shown that those filiform processes of the ectoderm of 

 the Htduoida, which he calls " palpocils," are composed of a true rhizopodal sarcode ; and these 

 processes are probably offsets of a very thin sarcode layer, which, as already mentioned, can be 

 seen, in certain cases, to extend over the surface of the hydranth. 



Under the head of contractility the phenomena of ciliary motion must also be included. 

 Vibratile cilia, as we have already seen, exist in almost every case on the walls of the somatic 

 cavity, and mainly contribute to the production of the currents so well known in the nutritive 

 fluids of the Hydroida ; while the planula or early locomotive stage of most hydroids, as well as 

 certain hydroid medusae {Trachynemd) and the free sporosac of Dicori/ne cojiferta, are provided 

 with an external covering of vibratile cilia. 



5. Sensation. 



Of late years several observers have believed themselves successful in demonstrating a 

 nervous system in the Hydroida, while others have refused to admit the existence of a differen- 

 tiated nervous system in these animals, and consider the arguments which have been adduced in 

 favour of its presence as resting upon imperfect or incorrectly interpreted observations. 



The advocates for the existence of a specialised nervous system in the Hydroida all agree in 

 regarding as its principal part an apparent filament with ganglion-like enlargements, which may be 

 seen running in the form of a ring round the margin of the medusa just below the circular canal. 

 Agassiz maintains the existence of such a ring-like cord, and assigns to it the function of a nerve- 

 ring, in Sarsia, Tinropsis, Sfaurophora, and BoiicjainvilUa ; M'Crady in Euclmlota ; Fritz Miiller in 

 Tamoya, a genus belonging to the family of the Cliaryhdrnda, as well as in Liriope and Cunina, 

 true hydroids ;* and Leuckhart in a medusa which he refers to Gegenbaur's genus Eucope ; while 

 Hensen also admits the presence of a nerve-ring in the Eucope of Gegenbaur. But by far the most 

 complete description we possess is that by Haeckel," who has studied the structures in question 



' The hydroid structure of the (Eginida, in which Cunina is included, must follow, from the 

 remarkable observations of Haeckel on the structure of Cunina and on its genetic relations with the 

 Geryonidan Medusse. 



" Haeckel, op. cit. 



18 



