BIMERIA RIGIDA SP. N. 13 



arise from the body of the hydranth in the neighbourhood of 

 the tentacles. 



With these marked dissimilarities I am reluctant to refer 

 rigid a to the genus Sph^erocoryne. The capitula of 

 rigida are small although perfectly definite structures, and 

 it is easier to suppose that their production is associated with 

 the enclosure of the tentacles with the perisarc rather than 

 that it indicates a coryne-like ancestor. A character such 

 as the capitation of tentacles could, we may readily suppose, 

 easily originate independently several times in the hydroid 

 series. 



As far as can be judged from the published descriptions 

 of the species of Bimer ia, the perisarc-cup is closely adherent 

 to the ectoderm of the hydranth, but such is not the case in 

 B. rigida, for in the proximal region the body of the 

 hydranth is widely separated from it (PI. I, figs. 3, 9). 



The true calyx of the Calyptoblastica differs from the 

 perisarc-cup of the Gy mnoblastica in that typically in 

 the former the body of the adult hydranth is free from the 

 mouth of the calyx. The calyx is, as. a rule, relatively large, 

 and the hydranth can retract into it. In some Calypto- 

 blastica, however, as in Sertularella spp., Sertularia 

 s-pp., and Thyroscyphus spp., the calyx is lined by a layer 

 of ectoderm, which near the mouth-edge of the calyx may 

 bear a well-developed battery of nematocysts. The calyx is 

 secreted by the ectoderm of the hydranth-bud, and then, 

 subsequently, this ectoderm splits into an inner and outer 

 layer. The inner layer forms the ectoderm of the body of 

 the hydranth, and the outer layer, which consists of a thin 

 sheet of flat cells, foi-nis a lining to the calyx. The lining 

 ectoderm layer is by no means always present in the adult 

 calyx, as, for example, in Plumularia; but whether this 

 absence is due to the non-occurrence of the splitting of the 

 ectoderm of the hydranth-bud, or to the subsequent disap- 

 pearance of the outer layer, has still to be determined. 



A quite similar splitting of the ectoderm occurs in the case 

 of the gonangia of the Calyptoblastica. The ectoderm of 



