14 ERNEST WARREN. 



the gonangium-bud secretes the perisarc of the gonotheca 

 and afterwards splits, except in the region of the " Decken- 

 platte/' into an outer layer which lines the gonotheca and an 

 inner layer which covers the blastostyle. 



In the Calyptoblastica these accessory sheets of ecto- 

 derm may be present on the " dorsal " or abcanline surface 

 of the coenosarc of the hydrocaulus, and also around the 

 hydranth and gonangium. The layers are closely adherent 

 to the perisarc, and they must certainly serve to anchor the 

 living tissues to the supporting structure, and possibly this is 

 the main function. Quite likely the cells may be especially 

 contractile, and the so-called opercular muscles of the 

 hydranth appear to be portions of this sheet. 1 have had no 

 opportunity of observing whether these reputed muscles 

 actually serve to move the opercular flaps. In this connection 

 it may be mentioned that the lining sheet of ectoderm is well 

 developed around the hydranth of Sertularia acantho- 

 s torn a Bale, although there are no opercular flaps and the 

 margin of the calyx is dentate. 



These accessory sheets of ectoderm have not been found in 

 the coenosarc of any of the Gymnoblastica that I have 

 exainined, and their formation would seem to be a somewhat 

 chai-acteristic feature of the Calyptoblastica. Their 

 non-occurrence in Bimeria rigida serves to indicate that 

 this species, notwithstanding the presence of a kind of calyx, 

 is not necessarily to be regarded as a step in the direction of 

 the calyptoblastic hydroids. 



In Bimeria rigida practically the whole of the hydranth 

 is invested by a thick layer of perisarc, and this is closely 

 adherent to the ectoderm except at the base of the polyp. 



With respect to the perisarc it is very doubtful if Bimeria 

 rigida is to be regarded as especially intermediate between 

 the Gymnoblastica and Calyptoblastica, The calypto- 

 blastic condition probably arose from such a stage as is 

 exhibited by Coryne virgata Allman, where the 

 hydranth- bu.d is conspicuously enclosed by perisarc, and this 

 perisarc persists to an appreciable degree ai'ound the body of 



