CERTAIN" STICHODACTTLIKiE TO THE MADREPOEARIA. 649 



An ectodermal musculature has apparently never been 

 described in the Madreporaria, thougb the figure which Powler 

 .(1888, fig. 9) gives of SpTienotrochus rubescens suggests muscle- 

 fibres between the ectoderm and mesogloea. I have determined 

 its presence in the edge-zoues of at least two West-Indian 

 xjorals. 



Mesenteries. 



No distinction of any importance between the well-known 

 arrangement of the meseuterie-^ in Hexactinian polyps and that of 

 Madreporarian polyps has yet been advanced. Departures from 

 the hexameral symmetr'y are, however, numerous in both. Even 

 the same species of anemone may be hexamerous or octaraerous. 

 Thus McMurrich (1893) found Bahaman specimens of Aiptasia 

 annulata to be octamerous, while several Jamaican examples I 

 liave examined are all hexamerous. With regard to the order of 

 development of the pairs of mesenteries, a difference may also be 

 indicated in closely allied species of Actiniae. Some specimens 

 o( Aiptasia annulata which I have retain the '■' 2i!chvardsia-stage " 

 in the adult, so far as concerns the perfect mesenteries, the fifth 

 and sixth pairs (the sulco-sulcular and sulculo-sulcar laterals of 

 Haddon) to be developed never reaching the oesophagus. In 

 young examples of Aiptasia tagetes also only four pairs of the 

 first cycle reach the oesophagus, but the imperfect mesenteries 

 in this case are the innermost of each of the lateral pairs on 

 .each side (the sulco-sulcular and sulculo-sulcar laterals). We 

 have thus evidence that the bilateral and biradial types of Boveri 

 may be assumed in the same genus. 



Fowler has shown that directives are absent in the coral LopTio- 

 •helia, and Bourne in Mti^ssa and ^uphi/lUa, and the absence of one 

 or both pairs of directives is not uncommon amongst anemones. 



In a recent paper, " On some Irregularities in the Number 

 of the Directive Mesenteries in the HexactinisD," McMurrich 

 (1897) j)oi^ts out that the complete absence of directives is 

 not necessarily a phylogenetic peculiarity, and cites several 

 instances in which such occurs in widely removed species. 

 Similarly, an increase of directives beyond the usual two pairs is 

 occasionally observed. In the paper mentioned, the author 

 refers to seven examples of Sagartia spongicola, in six of which 

 ihis happens. 



In a West-Indian species of Pliellia, wdth eight pairs of perfect 



