THE ANATOMY OF THE MADREPOKARIA. 21 



The three polyps here quoted were within a few millimetres of 

 each other, and all were nearly of the same size. 



The number of pairs of mesenteries is of course the same as that 

 of the septa, the latter being entocoelic only, though a misleading 

 appearance of ectocoelic septa is produced by the fact that some pairs 

 of mesenteries die out after a very short course, while their septa 

 are still recognisable at a much greater depth in the polyp cavity. 

 The mesenteries with a longer course are in all respects perfectly 

 normal, and in my specimens bore huge ova, the structure and 

 relations of which call for no special comment (Fig. 3). The length 

 or shortness of the mesenteries appears dependent on no particular 

 system, such as has been observed in some other forms (3). 



The tentacles are probably entocoelic only, but are so retracted as 

 to render the point somewhat obscure. In this condition they are 

 covered by a ring-fold formed of the indrawn margins of the disc, a 

 method of protection common among the Actiniaria. 



The histology, though much spoilt by prolonged decalcification, 

 agrees with that of the typical forms already described. The 

 muscular pleats of the mesogloea of the mesenteries are only very 

 slightly and irregularly developed, but entirely normal. Nemato- 

 cysts are closely packed together in the tentacles ; they are not, 

 however, arranged in knobs or " batteries." 



Zooxanthellse are present abundantly in the canals exterior to the 

 theca, in the tentacle cavities, and immediately under the mouth 

 disc ; elsewhere they are comparatively rare. 



in. Summary. — The following are the most important points 

 elucidated : — 



1. The polyps are of the normal Actinian type, and are bilateral, 

 but not rigidly bisymmetrical. 



2. The septa and tentacles (1) are entocoelic only. 



3. The number of septa present is inconstant, and bears no relation 

 to any multiple of six. 



4. The general body wall of the colony is supported upon the echinu- 

 lations of the comenchyme; a condition which may be of secondary 

 significance, acquired for the purpose of such support, contempora- 

 neously with and in consequence of the development of ccnenchyme. 



