-114 GEORGE MATTHAI 



layer, part of it arises in the encloclerm, viz. tlie median core of 

 mesenteries, and the remainder is contributed to by both the 

 ectoderm and endoderm, viz. the middle lamina of the column- 

 wall, oral-disc, edge-zone, and stomodaeum. While Bourne 

 in 1899 held that the processes of attachment, which were 

 essentially similar to and became part of the middle lamina, 

 were formed by modification of elements in the calico blast ic 

 layer or ' desmocytes ', i.e. were intra-protoplasmic formations, 

 he had in a previous paper (5) inferred that the middle lamina 

 itself was a secretion of ectoderm and endoderm, i.e. was an 

 extra-protoplasmic product. 



The middle lamina appears to be essentially a supporting 

 stratum, i.e. has the function of connective tissue of Vertebrates 

 and, like the latter, has a fibrous texture. It is best developed 

 in mesenteries, since they support the oral-disc mth the ten- 

 tacles and keep the stomodaeum in position, the longitudinal 

 pleats giving additional strength to the mesenteries. Owing 

 to the presence of a calcareous skeleton to support the colunm- 

 wall, the middle lamina in the Madreporaria is very thin, 

 whilst in Actinianstheabsence of such a skeleton has necessitated 

 a considerable thickening of the middle lamina in the column- 

 wall (being best developed in this region), which, when the 

 column- wall is folded longitudinalh* as in Metridium 

 senilis, is swollen into longitudinal ridges within the folds 

 or rugae (fig. 16). The column-wall processes are analogous to 

 tendinous structures in Vertebrates, since, doubtless, they 

 attach the soft parts to the corallum. This function would 

 account for their sucker-shape, usually concave attaching 

 surface, and comparatively small size, combined with their 

 numerical abundance ; the specialized fibres in them presum- 

 ably impart additional toughness to these processes. 



Since the middle lamina has a spongy texture, the infilling 

 of its meshes with fluid would help in the distention of polyps, 

 which is, however, mainly effected by the ingress of sea-M^ater 

 into the polyp cavities, while the general contractility of the 

 middle lamina would help in the retraction of polyps. 



