108 GEORGE MATTHAI 



subsidiary mesenteries) are similar to the inner lining of its 

 stomodaemn, i.e. nuclei in them are not so closely aggregated 

 nor so slender as in filaments of polyps. 



It is obvious that in a subsidiary mesentery of a polyp 

 the filament is formed by modification of the endoderm of 

 the mesentery along its free margin, attaining histological 

 similarity with the inner lining of the stomodaeum and with 

 filaments of principal mesenteries. Stages in this modification 

 are seen in subsidiary mesenteries of varying width. In larvae 

 of F a V i a f r a g u m , also, filament-epithelium is present along 

 the margins of some subsidiary mesenteries which is undoubtedly 

 formed by modification of the marginal endoderm of those 

 mesenteries. I have previously described the presence of 

 filament-epithelium on the mesenteries of an extra-tentacular 

 bud of Fa via hululensis (Gard.) (27). 



In some species, F a v i a f r a g u m , M e r c i 1 i n a a m p 1 i a t a , 

 H y d e r o p h o r a m a 1 d i v e n s i s , I s o p h y 1 1 i a d i p s a c e a , 

 there are regions in the convolutions of mesenteries in which 

 the filament-epithelium is considerablyvacuolated and swollen, 

 in which nematocysts i or ii are closely arranged. In dumb- 

 bell-shaped transverse sections of these, the filament-epithelium 

 at each end resembles the intervening endoderm. In other 

 words, histologically identical epithelia are found in stomodaea 

 and mesenteries, whether the inner linings of the former and the 

 filaments of the latter are ectodermal or endodermal in origin. 

 But it is to be noted that algae are absent from the inner 

 lining of the stomodaeum and mesenterial filaments. 



The Supporting Middle Lamina. 



The middle lamina is found everywhere between the ectoderm 

 and endoderm and forms the median core of mesenteries. 

 Though Bourne could find no trace of structure in the middle 

 lamina of F u n g i a , he remarked that the use of proper 

 reagents might possibly have disclosed a fibrillar structure 

 (5, p. 310). As a result of making careful microscopical prepara- 

 tions this lamina is now found to consist of (1) a homogeneous 

 matrix or clear cementing substance containing (2) fine fibres 



