MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 477 



The structural uniformity of the filaments in all Madreporaria, and the absence of lateral 

 lobes bearing ciliated bands, simplifies the matter somewhat in this group, as compared with the 

 Aetiniaria. From its relationships to the mesentery, its form and histological structure, the 

 Madreporarian filament for the greater part of its course undoubtedly corresponds with 

 the middle lobe of the Aetinian filament. In the simple condition of the latter, the organs are 

 Indistinguishable in the two groups, and without doubt a common phylogenetic origin must 

 be assigned them. 



In connection with the origin of the filaments, II. V. Wilson was the first to attach impor- 

 tance to a reflection of the stomodseal ectoderm, which takes place at the inner termination of the 

 stomodseum. This occurs in both adult and larval polyps. On PI. VI, fig. 51, representing a 

 transverse section through the terminal stomodseal region of an adult polyp of Cladocwa, the 

 ectoderm is seen to line not only the outer surface of the wall, but has also become folded round 

 the edge of the stomodseum, and comes to occupy the inner or endodermal surface of the 

 organ, thence passing for some distance along both faces of the complete mesenteries, and 

 seeming to give rise to the mesenterial filaments as the mesenteries become free. Similarly, on 

 the left side of fig. 56. a vertical section through the stomodajum of C 'ladocora, the ectoderm 

 becomes folded at the termination of the wall, and is then continuous with the mesenterial 

 filament. The right side of rig. 2 shows the same relationship in Madrt/pora. 



On PI. XIV, rig. 112. a transverse section through an early larva of Favia, also displays a 

 tissue on the endodermal surface of the stomodseum, in all respects resembling that of the 

 stomodanil ectoderm. Separated by the first pair of mesenteries, it forms a distinct, deeply- 

 staining lobe, in both the larger and smaller primary chambers, and differs greatly from the 

 surrounding endoderm. In sections a little higher, the reflected ectoderm is wholly wanting, 

 and the stomodseal lining is purely endodermal (cf, figs. 126, 127). 



The stomoda>al ectoderm, reflected in this way around the lower edge of the stomodreal wall, 

 occurs to a greater or less degree in probably all Madreporaria. Invariably, the mesenterial 

 filaments of the complete mesenteries seem as if they took their origin from it, and the histo- 

 logical resemblance is very close. The extent of the reflection along the ccelomic surface of the 

 stomodieum, and also outwardly along the mesenterial faces, varies much with the amount of 

 retraction or expansion of the polyps, for in the latter condition the stomoda>al wall and edge of 

 the mesentery come to he almost in the same vertical straight line, and no reflection is then 

 apparent. 



Wilson, in his studio of the early larvae of Manicina, found the stomodseum to be applied to 

 the column wall, and its ectoderm appeared to pass down the inner surface of the wall, even before 

 the first pair of mesenteries had appeared. In later stages the ectoderm of the stomodseum was 

 reflected up the endodermal surface, and all the primary mesenteries, except the first pair, were 

 considered to receive their filamental tissues from these ectodermal tracts, though some filaments 

 were found to be present on the mesenteries before the union of the latter with the stomoda?um 

 had been effected. With regard to the origin of the filaments on the mesenteries which never 

 reach the stomodseum, probably few students of the embryology of the Anthozoa will be 

 prepared to follow Wilson in his suggestion (p. 22<») that these receive their ectoderm from 

 a reflection along the entire length of the coelomic surface of the stomodseum and peristome 

 (the epithelium ./■. of Wilson's figs. 50 and 55). Its acceptance, in the case of the incomplete 

 mesenteries of some species, would demand that the inner lining of nearly the whole of the 

 upper region of the polyps should consist of ectodermal tracts. 



.V typical example of the condition of the filaments found in larva' is presented by the 

 transverse sections of the larva of Favia fragurn represented on PL XIV. For the greater 

 part of its length the inner (ccelentaric) layer of the stomodseum resembles the rest of the endo- 

 derm. but toward the internal end it bee-ins to assume a histological character more nearly 

 resembling that of the ectodermal lining. The cells are now narrow and closely arranged, and 

 the numerous brightly-staining nuclei form a definite zone, marking off the region very distinctly 

 from the ordinary endoderm. This is the so-called " reflected ectoderm "' of Anthozoan literature. 

 In Favia it passes along the first pairs of mesenteries for a short distance, and as these sever 

 Vol. s— No. 7 6 



