XVI INTRODUCTION. 



doubt, largely instrumental in promoting the aeration of 

 the fluids which enter them freely from the perigastric 

 cavity. They must also be regarded as tactile organs, 

 and in many species are furnished with special appen- 

 dages, by which their sensitiveness and power of detecting 

 the presence of minute particles arc largely increased. 

 They are (as Hyatt has remarked *) " the only means 

 possessed by the polypidc of receiving impressions from 

 without." 



The energetic movements of which the tentacles are 

 capable would lead us to expect an efficient service of 

 muscles in connexion with them. In Laguncula (Far- 

 rella) Van Beneden describes muscular fibres in each 

 arm ; he also detected delicate bands in the freshwater 

 group which he regards as of the same nature. Hyatt 

 has demonstrated two sets of " tentacular bands," an 

 outer and an inner, in each arm, which are in connexion 

 with muscles running through the lophophore, and con- 

 trol the movements the external bands inclining the 

 tentacle outwards or sideways, the internal bending it 

 towards the centre of the lophophore. This observation 

 has been confirmed by Nitsche, who finds in the ten- 

 tacles a pair of fasciculi, each of which consists of two 

 or three long fibres. 



In the tentacles of Membranipora membranacea and of 

 ' Pedicellina the latter author has noticed obscure traces 

 of muscular structure ; those of Loxosoma, according to 

 Vogt, show no indication of it. 



* Op. cit. p. 47. The polypides (according to this author) are not 

 sensitive to light. " At ordinary temperatures even the darkness-loving 

 Fredericella may be exposed for a time to the direct rays of the- sun without 

 any visible result, although but just remou-d from the perpetual shade in 

 whidi it had previously lived." 



