BIMERIA RIGIDA SP. N. 15 



the adult hydranth. A similar envelope persisting around a 

 modified person producing a cluster of gonophores, such as 

 the blastostyle of Heterocordyle, would clearly give rise to 

 the gonotheca of the Calyptoblastica. 



In Bimeria rigida it is quite certain that the sporosac is 

 a reduced medusa and not a blastostyle producing one gono- 

 phore, for in the i-elated Parawrightia well-formed radial- 

 canals are present in the structure. Thus^ notwithstanding 

 the development of a calyx, Bimeria rigida must be 

 regarded as a true gymnoblastic hydroid. The development 

 of perisarc is a means of defence, and in this species we reach 

 what would appear to be the maximum development possible 

 in the Gymnoblastica. 



From such a condition as is seen in Coryne virgata we 

 pass on the one hand to Bimeria rigida of the Gymno- 

 blastica and on the other hand to the operculate forms of 

 the Calyptoblastica. 



It can be i-eadily understood that a thick layer of perisarc 

 closely investing the body of the hydranth is bound to inhibit 

 movement to some extent, and to prevent a rapid response to 

 an external stimulus notwithstanding the occcurrence of 

 especially flexible perisarc on the hypostome region and on 

 the oral face of the tentacles. In the case of the Calj^pto- 

 blastica the hydranth when extended from the cup has the 

 advantage of perfect freedom of movement, while rapid with- 

 drawal into the calyx provides a very efficient protection 

 against enemies. 



Finally, it may be said that in B. rigida we reach the 

 extreme type of protection by perisarc production in the 

 Gymnoblastica, and this apparently lias not proved a great 

 success, since the condition is rare or almost unique. The 

 formation of a true calyx free from the body of the 

 hydranth and into Avhicli the hydranth can be withdrawn and 

 enclosed by opercular folds, or protected by overhanging 

 teeth, has clearly proved a more eifective method of protec- 

 tion, since calyx-bearing hydi'oids are the dominant forms of 

 the present day. 



