740 THE ANIMAL KINGDOM. 



species are in the young condition very closely alike, and sometimes indis- 

 tinguishable ;' and 'in the very early stages the young of even widely dif- 

 ferent species of the same genus are almost absolutely alike ' (Moseley), 

 facts especially noted in the genus Flabellum. Tabulae occur in Pocillopo- 

 ridae and a few Poritidae. 



As to the zooids they are, in the majority of instances properly ex- 

 amined, Hexactinian in structure. The tentacles do not appear to be 

 perforated by an apical pore. They may correspond to the intraseptal 

 chambers alone, e. g. in Rhodopsammia parallela, or to the interseptal as 

 well, the more usual arrangement. They are simple in form and often 

 armed with batteries of aggregated nematocysts. The mesenteries may 

 be only twelve in number, all complete (Stylop/wra, Madrepora), or, when 

 the calycles are large or the animal solitary, much more numerous and 

 some complete, others incomplete. They are disposed in the typical 

 Hexactinian mode. The septa are contained only in the intraseptal 

 chambers, or in the interseptal as well. Septal stomata have not been 

 described nor Rotteken's sphincter muscle. Sexual organs may be present 

 on all the mesenteries, or confined to those of a certain order, or, as in 

 Madrepora Dtirvillei, to two specially elongated mesenteries. The sexes 

 appear generally to be separate even in the colonies. In the coral last 

 named a remarkable dimorphism occurs between the zooids in respect of 

 the mesenteries 1 . 



Seriatopora differs in several respects from typical Madreporaria. Its 

 calycles are elongated and disposed lengthwise on the branches of the 

 coral, and are traversed by a longitudinal plate. There are twelve tentacles 

 in two cycles, and twelve mesenteries, two of which, towards the basal or 

 ventral end of the calycle, are of great length, and alone have mesenterial 

 filaments and genital organs. The chambers to which they belong are 

 greatly prolonged and received into deep pits in the calycle. There are 

 three complete septa and two rudimentary on each side. The form and 

 arrangement of the mesenteries and septa bring about a marked difference 

 between the two ends of the zooids or calycles. The colony is unisexual. 

 Pocillopora closely resembles Seriatopora. 



The zooids in a colony are either isolated, e. g. Cladocora, or where 

 there is a coenosarc, they are connected by a superficial canal system as in 

 Stylophora, Seriatopora and Pocillipora, by a deep system as well in Madre- 



Fungia the synapticulae or plates connecting the septa commence as points on the opposing surfaces 

 of two adjoining septa, which grow, meet and fuse, perforating the intervening mesentery in the 

 process. The edge of the flattened theca grows similarly ; an extrathecal space underlies it, divided 

 into radial chambers by the aboral continuations of the mesenteries and connected to the intrathecal 

 space peripherally as well as by canals passing through the theca itself. 



1 Fowler states (Q. J. M. xxv. p. 588) that there are acontia (?) and peristomial cinclides in 

 Flabelhim patagonicum. Mr. G. C. Bourne tells me that he has observed the protrusion of much 

 convoluted mesenteries from peristomial cinclides in Maeandrina. 



