MEMOIES OF THE NATIONAL ACADEMY OF SCIENCES. 509 



lar disk, surrounded by a single system of tentacles; and in the differenl polyps of any colony 

 all stages can be traced in the separation of the results of fission. Ii is therefore a very favor- 

 able species for the study of polypal fission. <i. von Koch (1890) lias already described the 

 process ;i s it occurs in the corallum of the nearly allied species, Yamia ca/oemosa. 



Fortunately, also, in F. fragum a complete series of stages illustrating the development of 

 the mesenteries within the larva and young poly]! is available, a series extending from the larva 

 with only one pair of complete mesenteries to polyps with such a number as results in fission. 

 The oldest sta.e-e reached by the mesenteries of the simple polyp is represented in tie-. i;„/. but the 

 earlier stages may be briefly noted (tigs. 15 (»-/'). 



The twelve primary mesenteries are all developed at or shortly after the time of settling. 

 when the larva exhibits the conditions represented in tig. 15(7, four pairs of mesenteries complete 

 and two pairs incomplete. Free swimming larva', extruded from mature polyps, are readily 

 obtained, but can with difficulty be induced to settle, and development proceeds very slowly. 



I 



Fig. 15c. 



Ftovia fragum.— Young polyp with four pairs of metaenemes (A, A; B, B). The succession of the second-cycle mesenteries is bilateral, from 



the dorsal to the ventral aspect (cf. PI. xiv, fig. 109. ) 



However, on foreign objects, such as dead coral or old shells, to which the mature colonies are 

 adherent, young polyps are sometimes found in different stages of development. These have 

 grown from larvae which on extrusion settled around the parent, and it was from such larval polyps 

 that the stages represented in the text figures were taken. 



Fif. 15'- is from a young polyp in which four isocnemic pairs of mesenteries are present, in 

 addition to the primary twelve. In the upper stomoda?al region all the latter are inserted on the 

 stomodseal wall, but the fifth and sixth pairs become free before the termination of the stomo- 

 danim is reached, and at this place the protocnemes are in exactly the same condition as in fig. 15c/. 

 The four pairs of new mesenteries are situated within the dorsal and middle primary exocoeles 

 on both sides of the polyp, and the dorsal pairs are somewhat further developed than the mid- 

 file pairs. 



Vol. 8— No. 7 8 



