THE ANATOMY OF THE MADREPORARIA. 27 



pairs* on the normal type. In the diagram (Fig. 9) they are 

 numbered in the same manner as those of Madrepora (3) ; the two 

 mesenteries marked 3 and 10 respectively are comparatively long, 

 extending to the bottom of the polyp cavity, and possess the 

 thickened edge known as a mesenterial filament ; of the rest, those 

 numbered 1, 5, 8, 12, though generally devoid of a " filamentar " 

 thickening, are recognisable in transverse sections for some distance 

 below the stomatodasum ; while the others, 2, 4, 6, 7, 9, 11, are 

 rudimentary, and are visible only in the highest sections. It is 

 worthy of remark that the six rudimentary mesenteries last men- 

 tioned are those which in the one type of polyp of Madrepora 

 Dunillei are pierced by a special ectodermal canal, and which in the 

 other type of polyp of the same species, and in all the polyps of 

 M. aspei'a, are distinguished from the remaining six by a greater 

 length and the possession of a filamentar thickening ; in other words, 

 of the total twelve mesenteries the six which in the one form are the best 

 developed are in the other quite rudimentary. 



The histology agrees with that of the normal types, I have found 

 no trace of generative organs in my specimens. 



in. Summary. — The interesting points in Seriatopora are : — 



1 . The polyps are Actinian in structure. 



2. The septa, when all are present, and the tentacles, are both 

 ectoccelic and entoccelic. 



3. The tentacles are retracted by introversion. 



4. The body wall is supported upon the echinulations of the 

 ccenenchyme. 



5. Of the twelve mesenteries, six (and more especially two of 

 these) are of some length, and six are rudimentary ; but those which 

 here are well developed are, in the Madreporae mentioned above, 

 rudimentary, and vice verstl. 



* Professor Mpseley CIO) states that in Seriatopora and Pocillopora the mesen- 

 teries " are Dot di sposed in pairs with regard to the septa," and the remark appears 

 in a mialAn/ting form in Professor Martin Duncan's " Revision of the Madreporaria " 

 ("Journ. Linn.8oc.Zool.," Vol. xviiL), to the effect that "the genera differ from other 

 Madreporaria in not having tluir mesenteries arranged in pairs." The original 

 ..,;, , correct, because the possibility of ectoccelic septa in a coral had not 

 been demonstrated. 



