THE ANATOMY OF THE MADREPORARIA. 5 



pairs of " directive " mesenteries, distinguished by the arrange- 

 ment of retractor muscles on their ectoccelic faces. The 

 polyps are not, however, rigidly bisymmetrical, inasmuch as 

 the pairs of mesenteries lying right and left of the dividing 

 plane are not equal in number. 



The total number of pairs of mesenteries is not constant, 

 but does not appear to depend upon the size (age) of the parti- 

 cular polyp. It varies generally from 17 to 22. The asym- 

 metry of the polyps can best be seen in a tabular form : 



Number of pairs of mesenteries . 

 Number ou right side of " directives " 

 Number on left side of " directives " 



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 ectoccelic 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 Acti- 

 niaria. 



The histology, though much spoilt by prolonged decalcifica- 

 tion, agrees with that of the typical forms already described. 

 The muscular pleats of the mesoglcea of the mesenteries are 

 only very slightly and irregularly developed, but entirely 

 normal. Nematocysts are closely packed together in the 



