THE ANATOMY OF THE MADREPORARIA. 11 



they are simply downward continuations of the conical ccelen- 

 teron, and mesenteries other than the two longer ones are 

 sometimes attached to their sides. Other details of the 

 skeletal structure do not especially bear on the anatomy of the 

 polyps. 



ii. Anatomy. — As was shown by Professor Moseley, Seria- 

 topora is undoubtedly a Madreporarian, and is even more in 

 accordance with the normal types than could be inferred 

 without the aid of sections. 



The whole of the colony is clothed in the customary body 

 wall of ectoderm, mesogloea, and endoderm (fig. 9), which is 

 supported on the echinulations of the ccenenchyme (vide 

 supra, p. 3). The space between body wall and theca is 

 broken up by these spines into a superficial series of canals 

 (figs. 9, 10, 13), which ramify over the ccenenchyme and place 

 the polyp cavities in communication with each other, but do 

 not, of course, extend into the corallum in the manner cha- 

 racteristic of the Perforata. The body wall is continuous with 

 the mouth disc, and. from the centre of the latter rises a slight 

 hypostome, through which opens the stomatodaeum. This 

 latter is crucial in transverse section, the longer arms of the 

 cross being in the dividing plane of bilaterality indicated by 

 the axial and abaxial septa (fig. 10). 



The tentacles, which are twelve in number, being both ecto- 

 coelic and entocoelic, are simple evaginations of the ccelenteron, 

 tipped with a terminal swelling, which is a single " battery" 

 of nematocysts fig. 11). There is, I believe, no instance yet 

 recorded of the occurrence among Madreporaria of the method 

 of tentacular retraction which distinguishes Seriatopora, 

 namely, that of introversion (figs. 12, 13), the tentacles 

 being invaginated in such wise that the battery is still pointed 

 upwards. In fig. 13 the ectocoelic tentacles are expanded, 

 while the entocoelic are introverted, a condition not uncom- 

 mon in my specimens. Probably owing to the minuteness of 

 the polyp, no special muscular apparatus for effecting this 

 retraction could be detected. 



The mesenteries, which are twelve in number, are arranged 



