BIMEHIA KIGIDA SF. N. / 



This axial rod is nearer the upper than the under surface, 

 so that in cross section it is very excentric in position (fig. 16, 

 e)i.). Owing to the thinness and soft nature of the perisarc 

 which covers the upper surface, the tentacle can bend towards 

 the mouth. This bending would be scarcely possible if the 

 thick, firm perisarc formed a complete tube around the tentacle. 

 The ectoderm of the tentacle consists of peculiar, elongated, 

 thread-like cells (figs. 18, 19, 20, m. e. t.) which are doubtless 

 especially muscular in nature, and this great development of 

 muscle is clearly correlated with the fact that considerable 

 muscular power is required for bending the firm perisarc of 

 the under sui'face. 



The capitulum of the tentacle is provided in a typical 

 manner with large, elongated nematocysts (fig. 19, n.). These 

 are doubtless of use in securing prey. The similar nematocysts 

 which occur in the ectoderm enveloped by the calyx and the 

 perisarc of the hydi'ocaulus cannot be so utilised, and, if of 

 any functional use, they can only serve to render the hydroid 

 disagreeable in being eaten as a whole. 



The food obtained by the hydranth appears to be of the 

 usual kind ; remains of small crustaceans, etc., being found in 

 the coelenteron. The external aperture of the apical ectoderm- 

 chamber and the mouth proper can be very greatly extended, 

 so that the endoderm may be widely exposed to the sea- 

 water and relatively large prey can be swallowed (PI. I, fig 9). 



The function of the ectodermal chamber is obscure. The 

 chamber is not an accidental or occasional formation, but it 

 is a perfectly definite sti^icture which occurs in all the 

 hydranth s. It certainly forms a very efficient protection to 

 the mouth, and would render it exceedingly difficult for 

 parasites, such as pycnogonids, to enter the coelenteron when 

 the hydranth is at rest, or when the mouth is slightly opened 

 for taking in water. 



It may be noticed in this connection that the entire hydrantli 

 is singularly well protected from possible enemies by means 

 of the great development of the perisarc. It may be further 

 noted that we have only to suppose the ectodermal chamber 



