7. Anthozoa (incl. Hydrocorallia). A. Zoantharia. 21 



disposition is never assumed ; the directives form isocnemic pairs but the other 

 pairs are all anisocnemic. The pinnate method of mesenterial increase is 

 distinct from that characteristic of most recent corals ; in these the metacnemes 

 are added in isocnemic pairs all round the periphery of the polyp, within the 

 6 primary exocoels and finally constitute one or more distinct alternating cycles. 

 The method of mesenterial increase beyond the protocnemic stage occurring 

 in Madrepora is also characteristic of Porites except that in any one polyp of 

 the latter the new mesenteries are disposed at either the one or the other 

 extremity, not at both. Six new bilateral pairs of mesenteries appear practi- 

 cally simultaneously in M. , but only later do they all extend down the wall 

 of the polyp. In P. the new pairs follow one another in a regular succession. 

 In M. the mesenterial increase is early associated with fission of the stomo- 

 dseum and in the end probably with complete fission of the polyp. Half the 

 mesenteries of each fission polyp are derived from the primary 12 of the 

 original polyp, the other half are new formations. The resulting paired ar- 

 rangement of the mesenteries, including the presence of 2 pairs of directives is 

 exactly as in primary polyps. Fission of the stomodseum appears very late 

 in P., not until after the 6 new pairs of mesenteries are fully established. 



DuerdeiH 4 ) describes the formation of colonies in Siderastrcea radians by 

 fusion of originally free, distinct larvse. Colonies of S. are occasionally met 

 with in which the coelentera of many of the polyps are crowded with Iarva3. 

 These are shot out from time to time, sometimes only one or two but at 

 other times as many as 20. By the evening of the second day after extrusion 

 some of the larvae have become fixed either isolated or in small groups the 

 individuals of which are so close that the basal part of the column of some 

 of them is distorted by mutual pressure. The isolated young polyps deve- 

 loped regularly. The 6 pairs of primary mesenteries were followed in about a 

 month by members of the second cycle, the 6 primary tentacles (exocoelic) 

 which appeared two or three days after fixation were also followed in about 

 a mouth by the 6 of the second cycle (entocoelic). Three or four days after 

 fixation the 6 primary septa (entocoelic) appeared and a day or two later the 6 

 exocoelic septa began to appear in bilateral pairs from the dorsal to the ventral 

 aspect of the polyp. A thin basal plate and an epitheca were also formed. 

 Such a regular and symmetrical development was characteristic only of isolated 

 polyps. Those crowded together in groups had to arrange themselves at all 

 angles with regard to one another and to the object on which they were fixed. 

 All were normal with regard to the occurrence of the 6 pairs of primary 

 mesenteries and the 2 cycles (of 6 each) of tentacles. Variation and irregu- 

 larities arose in connection with the formation of the skeleton. The out- 

 line of the epitheca was irregular and in one group of 17 not one polyp 

 showed the two orders of 6 septa fully developed. Any number of septa from 

 4-12 was found, large and small ones as a rule alternating. Growth of such 

 polyps was slow compared with that of isolated polyps. In the later stages the 

 primary irregularities due to crowding would probably have been outgrown and 

 the groups would then present all the characters of normal colonies produced 

 by budding. On colonies of Manicina areolata and Favia fragum young 

 polyps are frequently found. These have settled, either immediately or shortly 

 after extrusion, upon some part of the parent colony. A similar aggregation 

 is described in Actiniaria. On the coral reefs in West-Indian waters the author 

 has observed large irregular patches, often several feet across, of Actinotryx 

 sancti-Thomce and Ricordea florida. In a single patch there may be hundreds 

 of individuals. Although both species show asexual reproduction by fission 



