SOLITARY CORALS. 227 



(Balanophyllia) ; the number of mesenterial pairs is very variable and there are no 

 directive mesenteries ; the tubes leading into the Aspidosiphon chamber are lined by 

 ingrowths of the body-wall consisting of ectoderm, mesoglcea, and endoderm. No 

 account is given of the histology. 



IT. mvltilobata is a well-marked colonial form with several calices, whereas 

 //. michelini usually lias two calices only, and some of the differences between my 

 observations and Fowler's are no doubt attributable to the difference in habit of the 

 two species. In older specimens of H. michelini the septal arrangement is sometimes 

 very irregular, but in young specimens with a single undivided calicle the septa are 

 arranged in a normal manner in six systems and four complete cycles, and their 

 relations are practically identical with those described in Balomophyllia, and with a 

 little trouble one may select a considerable proportion of adult specimens, hi which 

 two calicles have been formed by fissiparity, in which the septal arrangement differs 

 very little from the normal. 



The most interesting results, however, are obtained from specimens with a single 

 elongated but as yet undivided calicle. I made a series of sections through one such 

 specimen and another series through an obviously regular calicle of a specimen in 

 which fissiparity was complete. The sections show that the septa are alternately 

 exocoelic and endocoelic. In the first specimen with an undivided calicle there are 

 two pairs of directive mesenteries defining the primary septa at each end of the long 

 axis of the calicle. A portion of one side of the specimen was destroyed by the 

 ravages of a boring sponge, but in the complete half I was able to count three 

 systems and twenty-six endosepta arranged in four complete cycles, and a single 

 septum of a fifth cycle in the chambers adjoining the directive septa. I have given a 

 diagram of this specimen in fig. IV. The diagram is carefully constructed with 

 reference to a camera drawing of the actual section, but the complexity of the actual 

 drawing is so great, owing to the porous nature of the corallum, and the details are 

 60 minute that it is impossible to get them all into any figure of reasonable size other 

 than a diagram. 



It will be observed that the arrangement of the septa and their relations to the 

 mesenteries are practically identical to what has been described in Heterocyathus. 

 But particular attention should be paid to the exocoelic septa, which are shaded with 

 lines to distinguish them from the endosepta. Nothing can be more clear than the 

 fact that their peripheral ends have been thickened ; that chambers have been formed 

 in the thickened ends, whereby the septa became Y-shaped. And, finally, that the 

 quaternary endosepta have been formed between the mesenterial pairs arising in the 

 forks of the endosepta, and in some cases they have and in some they have not united 

 with the inner ends of the exosepta. Moreover it is clear, from the manner in which 

 the inner ends of the exosepta unite in the columella, that the tertiary endocoeles 

 must have been formed in exactly the same manner as the quaternaries obviously 

 have been. There could not be a more striking demonstration of the validity of 



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