ACTINIARIA. 185 



mesenteries (formed later than the primary) in addition to the primary; these are 

 complete in most Hexadiniae (incomplete in some Sagartidae, Peachia, etc.) ; 

 there are six pairs of them, and they are placed in the intermesenterialfspaces, 

 the longitudinal muscular faces of each mesentery being turned towards the 

 corresponding face of its fellow of a pair, as in the case of the lateral primaries. 

 There may be, in addition, twelve pairs of tertiary mesenteries and twenty-four 

 quaternaries, and so on ; the mesenteries of each order being always in pairs and 

 placed in intermesenterial spaces. The mesenteries of each order are smaller 

 than those of the preceding order, and except in the case of the primaries and 

 secondaries do not, as a rule, reach the oesophagus. The space between the 

 mesenteries of a primary pair is a primary intramesenterial space, that between 

 the mesenteries of the next cycle a secondary intramesenterial space, and so on. 



The tentacles, like the mesenteries, are in cycles of different age, so that we 

 can distinguish tentacles of the first, second, etc. order. There is often a 

 corresponding distinction in size (Corallimorphidae), and the arrangement of 

 the mesenteries is reflected in that of the tentacles. In such cases the six 

 largest tentacles are over the primary mesenterial spaces ; the next six, which 

 are a little smaller, belong to the secondary intramesenterial chambers ; then 

 follow the twelve tentacles, still smaller, of the tertiary intramesenterial spaces ; 

 while the twenty-four last tentacles communicate with the intermesenterial 

 spaces. 



Acontia are present in the Sagartidae. The epithelial cells always carry 

 flagella or cilia. 



The sexes are usually separate, but a few are hermaphrodite. The generative 

 cells in some cases, if not all, escape through the gonidial grooves. 



Budding takes place in the colonial Zoantheae, and both budding and fission 

 are occasionally observed in some solitary forms (Anthea cereus = Anemonia 

 sulcata, Actinoloba, Actinia, and other genera). 



The development of mesenteries in some larval Actiniae is interesting in 

 view of permanent arrangements in the various tribes of the Actiniaria. It 

 is as follows : 



Stage 1. The two first mesenteries are at right angles to the long axis of the 

 oesophagus, and divide the coelenteron into two, generally unequal, chambers. 



Stage 2. The mesenteries of the second pair are in the larger of the two 

 chambers so formed. 



Stage 3. The mesenteries of the third pair develop in the smaller of the two 

 primitive chambers. 



Stage 4. The mesenteries of the fourth pair are within the unpaired chamber 

 enclosed by the mesenteries No. 2 (Fig. 149). 



Stage 5. Two pairs now arise simultaneously, and for some time remain 

 incomplete ; they are respectively between the first and second, and the first and 

 third ; their longitudinal muscles face the longitudinal muscles of the first and 

 second respectively. When these are completed we get the typical Hexactinian 

 arrangement of the above mesenterial pairs ; the third and second are directive. 

 Moreover, it is to be noted that Stage 4 exists permanently in the Edwardsiae, 

 while Stage 5 is found in Gonactinia and in a modified form in the ZoantJieae. 



Stage 6. A pair of small mesenteries with their muscles facing each other 

 appears in each intermesenterial chamber (exocoele), and so with subsequent 

 cycles. In Peachia only four pairs of secondaries are formed, those of the 

 dorsal intermesenterial chamber (opposite to the gonidial groove) being absent. 



The deep-sea forms are very commonly distinguished by the reduction of their 



