HISTOLOGY OF ASTRAEIDS 105 



polyp and in different species. In the tentacles it is consider- 

 ably swollen in some species, almosi; filling their lumina ; in the 

 column-wall it is thin in the stomodaeal region, but becomes 

 highly vacuolated and reticular below this region, where it con- 

 tains comparatively few nuclei which are arranged in a row near 

 the free surface ; in the stomodaeal wall the endoderm remains 

 uniformly thin. In the mesenteries the endoderm is usually 

 swollen along the pleatal region and l^ehind the filaments. 

 A narrow constriction is present behind the filament, which is 

 deeper in principal than in subsidiary mesenteries (figs. 7 and 8). 

 Numerous round bodies, usually regarded as symbiotic algae, 

 are present in the oral-disc, edge-zone, ard tentacular endo- 

 derm, i.e. in the exposed regions of colonies ; in some polyps 

 they are so massed as to fill parts of the endoderm. In 

 the mesenteries these algal bodies occur in varying numbers, 

 chiefly in the exocoelic side, but are scarce in the column -wall. 

 Fibrils are continuous between the endoderm and middle 

 lamina, somewhat as between the latter and ectoderm. In all 

 the larval stages examined, the endoderm has attained histo- 

 logical similarity with that of the polyp, but organic debris 

 containing scattered nuclei are still seen in the coelenteric 

 cavity (fig. 4) and are perhaps remnants of the contents of 

 the earlier solid planula stage (37, PI. ii, fig. 4). 



The distinguishing characters of the endoderm are its vacuo- 

 lated condition, presence of algal bodies, the numerical 

 inferiority and somewhat scattered condition of its nuclei. 

 The endoderm, unlike the ectoderm, has a homogeneous 

 appearance, except for its relative swelling in different parts 

 and the varying number of algal bodies present. 



Inner Lining of Stomodaeum (figs. 5 and 17). 



In the larva, the stomodaeum is said to be formed by 

 invagination of one of its extremities, while it is found that 

 in colony-formation new stomodaea may be formed by invagina- 

 tion or by the union of the broader mesenteries in diverti- 

 cula (30). In the two former cases, the inner lining of the 

 stomodaeum is a continuation of the surface ectoderm, while 



