448 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The adult polyps of < 'ladocora arbtcsctda always contain six pairs of complete mesenteries, 

 constituting a first cycle, and six alternating' pairs which remain incomplete and form a second 

 cycle. Representatives of a third cycle are usually developed, hut instead of consisting of twelve 

 pairs, one in each exoccele between the previous twelve pairs, only four or six pairs are usually 

 present, all on the same aspect of the older pairs (PI. VI. tig. 49). Earlier stages in the develop- 

 ment reveal that this is probably the sulcar aspect of each system (p. 458). 



In Siderastrsea radians six pairs of complete mesenteries are present, along with six 

 alternating incomplete pairs, and a few pairs belonging to the third cycle may also occur; 

 usually the third-cycle pairs are radially shorter than those of the second cycle, but at other times 

 they nearly equal them in size. In the larger species, 8. siderea, though more members of the 

 third cycle are present, the whole twelve pairs necessary to complete the cycle are rarely present 

 (PI. XXIII. tig. L53). 



The polyps of the seven genera described, all produced asexually by the process of 

 gemmation, are thus characterized by the very regular disposition of the mesenteries in 

 alternating hexamerous cycles. The first and second cycles are fully developed in all the adult 

 polyps, while, the third cycle may be only partly formed, but so far as it goes the members 

 alternate regularly with the other pairs, according to the order of appearance established on 

 p. 45.") . /. seq. In all the polyps two pairs of directives occur in the hrst cycle. So far as the 

 mesenterial arrangement is concerned, there seems no difference between a polyp originating as a 

 bud and one derived from a sexually produced larva; both follow the normal hexactinian plan. 



Only the members of the first and second orders ever become inserted on the stomodaeum 

 in the species studied. The later orders never become complete, but retain a definite size 

 characteristic of the species. 



MESENTERIES IN GENERA REPRODUCING BY FISSION. 



The asexual reproduction of the following genera takes place mainly, if not entirely, by 

 stomoda?al fission: Agaricia^ Tsophyllia, Dichocasnia, Fa/via, Manicina, Mseandrina, and Colpo- 

 phyllia. In the first four the polyps so produced may become more or less distinct from 

 one another, each with its own system of tentacles and a column wall; in the remaining genera 

 the separation is incomplete, and meandering discal, tentacular, and columnar systems are pro- 

 duced in place of distinct polyps, and only exceptionally are transverse walls developed, 

 which separate one series of oral apertures from another. Sections have been made through 

 polyps of each of the above genera, and reveal a mesenterial arrangement very different from 

 that already described for genera where asexual reproduction by gemmation is the rule. 



Transverse sections through two polyps of Agaricia fragilis are represented on Pis. XXIV 

 ami XXV, while the arrangements of the mesenteries of two different polyps of Tsophyllia are 

 diagrammatically shown on next page. Fundamental differences are at once apparent, compared 

 with the mesenterial plans already described. No directive mesenteries occur in these nor in any 

 of the other examples studied Very rarely the number of complete pairs may be six, but is usually 

 irregular, while the incomplete mesenteries vary greatly in number, size, and relation to the 

 complete pairs. The hexameral plan is altogether departed from, and each stomodaeum may have 

 from ten to twenty-five complete mesenteries associated with it. A regular alternation of second 

 and third cycle mesenteries is found in only one or two places, as at the upper right-hand region 

 of fig. h; here and there a single unpaired mesentery may occur within an exoccele. Of the many 

 polyps of each species examined no two display exactly the same number and relationship of the 

 mesenteries. 



A like absence of hexameral. or any other, regularity occurs in the polyps of Dichocasnia 

 and Favia (Pis. X1I1 and XVI). Transverse sections reveal a variable number of pairs of 

 perfect mesenteries from four upward, according to the size of the polyp, while the 

 alternating incomplete pairs are rarely the same in number and size in any two exocceles, and no 

 directives occur. 



In sections of mature colonies of Manicina areolata only two series of mesenteries can l>e 

 generally distinguished, complete and incomplete, the latter rarely affording evidence of alternating 



