MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 477 



The structural iinifoniiity of the filaments in all Madreporaria. and the absence of lateral 

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

 Actiniaria. 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 Actinian 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, H. V. Wilson was the first to attach impor- 

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

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

 transverse section through the terminal stomodteal region of an adult polyp of C ladomra, the 

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

 the edge of the stomodwum, 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 stomodseum of Cladocora, the ectoderm 

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

 filament. The right side of fig. 2 shows the same relationship in Madrtpora. 



On PI. XIV, fig. 112, a transver.se section through an early larva of Favia^ also disi^lays a 

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

 stomodteal 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 .stomoda?al lining is purely endodermal {cf, figs. 126, 127). 



The stomodteal ectoderm, reflected in this way around the lower edge of the stomodteal 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 re.semblance is very close. The extent of the reflection along the ccBlomic surface of the 

 stomodffium, 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 stomodsval wall and edge of 

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

 apparent. 



Wilson, in his studies of the early larvte of 2Iuiiicina,io\\i\A the stomodteum 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 stomodaBum 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 

 Avere found to be present on the mesenteries before the union of the latter with the stomod^um 

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

 reach the stomodteum, prol)ably few students of the embryology of the Anthozoa will be 

 prepared to follow Wilson in his suggestion (p. 220) that these receive their ectoderm from 

 a reflection along the entire length of the c<jelomic surface of the stomodanmi and peristome 

 (the epithelium ;<■, of Wilson's tigs. 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. 



A typical example of the condition of the filaments found in larvie is presented by the 

 transverse sections of the larva of Faola fraguin represented on PI. XIV. For the greater 

 part of its length the inner (ccelenteric) layer of the stomoda'um resembles the rest of the endo- 

 derm, but toward the internal end it begins 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 nucl(>i form a definite zone, marking ofl' the region very distinctly 

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

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

 Vol. 8— No. 7 6 



