THE ANATOMY OP THE MADREPOEARJ A. 7 



occurs a swelling due to elongation of the endoderm cells_, 

 through which runs, in the mesoderm, a canal lined by ecto- 

 derm, doubled back on itself, and opening at both ends into 

 the stomatodseum, with the ectoderm of which its lining is 

 continuous. 



Of twenty-one polyps examined, seven present this modifi- 

 cation of six (and in all cases of the same six) mesenteries, 

 namely, those numbered 2, 4, 6, 7, 9, 11, according to the 

 method employed in the diagram ; the other six mesenteries, 

 1, 3, 5, 8, 10, 12, and all the twelve mesenteries of the other 

 polyps, are perfectly normal, and show no tendency to such a 

 modification. Were it possible to explain the sectional appear- 

 ances by a contortion of the mesentery, the regularity with 

 which it occurs would be sufficient proof that it is a definite 

 modification of structure, the parallel of which has yet to be 

 sought in the Anthozoa. 



The unmodified mesenteries in Type A, generally die out 

 before the plane of the opening of stomatodseum into coelen- 

 teron is reached, in transverse sections. If they present a fila- 

 ment, which is seldom the case, it is of the same character as 

 that figured (fig. 8.ll), i.e. identical with that of a modified 

 mesentery ; more frequently none is present, or at most a 

 slight endodermal swelling on the free edge. 



The mesenteries 4, 9, run very much deeper into the corallum 

 than the others. 



Type B, of about the same diameter as A, is of the normal 

 Actinian structure. The twelve mesenteries are simple, and 

 exactly like those unmodified in Type A. Most of them die out 

 after a very short course, but those numbered 2, 4, 6, 7, 9, 11, 

 on the same notation as in fig. 8, present a more developed fila- 

 ment than the other six, and extend further down into the 

 corallum, and of these 4, 9, have by far the longest course, and 

 are the only ones that bear ova. 



We have thus two distinct types of polyp, the one distin- 

 guished only for entire normality ; the other with a hitherto 

 undescribed form of mesentery. In both is observable a differ- 

 entiation affecting the same six mesenteries, exhibited in the 



