MB. H. M. BERNAUD ON EECENT POEITIDiE. 133 



be invaginated. In its earliest development a simple cup, it has 

 become complicated in various ways ; primarily, by the develop- 

 ment of radial infoldings of the stiff external wall, comparable 

 with the infoldings of the cbitinous cuticle of Arthropods : 

 secondarily, (1) by further complications of these infoldings so 

 as to form an intricate ' internal ' skeleton, which may render 

 the primitive external cup unnecessary, and hence lead to its 

 becoming vestigial ; (2) by a process of repeated sheddings of 

 the external hard secretions, and the formation of new ones 

 (dissepiments and tabulae) across and among the existing ' in- 

 ternal ' skeletal structures." 



Further work with Madreporarian skeletons has only con- 

 finned this generalization. One or two points, however, require 

 attention. In my former paper these septal infoldings were 

 likened to the apodemes of Arthropods, formed by the infoldings 

 of the chitiuous skeleton which sometimes, e. g. in the cephalo- 

 thorax of the Spiders, form together an elaborate internal frame- 

 work. "While this resemblance is structurally accurate, the 

 comparison must be received with caution. The apodematous 

 system of the Arthropods can be shown, even in detail *, to be 

 due, at least in their earliest stages, to muscular action, either 

 directly drawing in the chitin to which it is attached, or causing 

 deep wrinkles or folds across the line of the muscles. But it is 

 difficult to see how the infoldings of the calcareous exoskeleton 

 of the early Madrepores to form septa could have been due to 

 muscular action. Dr. von Koch (I. c.) thinks that the septa 

 might have arisen in connection with certain endodermal ridges 

 found in some larvae. But we shall be probably nearer the truth 

 if we can find a cause for them in the ectoderm itself. Until 

 recently I thought that they were due to increased local activity 

 in the secretion of calcareous matter, which would therefore push 

 in the body-wall. From this point of view I found fault (Geol. 

 Mag. 1897) with Miss Ogilvie's description of the process as an 

 " invagination which became filled up " with skeletal matter. 

 But this terminology, though not felicitous, need not be alto- 

 gether wrong. It seems to me not unlikely that the puckering 

 which gave rise to the septa was caused by the growth of the 

 basal, and probably best nourished, wall of the polyp, and that 

 this wall, cramped by the primitive exoskeletal epitheca, could 



* Ann. & Mag. Nat. Hist. (6) x. p. 67 (1892). 



