MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 481 



a batch of larva' of Facia fi'uguni, only six primary tentacles appeared siniultaneou.sly, about 

 four days after the larvae were set free, and in other larva3 reared later the members of the inner 

 cycle appeared in advance of the outer (PI. XIV, figs. 106, 107). 



So far as I can disco\'er, Sidi:ra><trxK radians is unique among both corals and Actinians in 

 that the first tentacles to arise are the six exocoelic members. This relationship was established 

 in scores of instances, and no exceptions whatever were observed, so that it must be regarded as 

 characteristic of the species. The six members were developed sinuiltaneously a few days after 

 fixation of the larva?, and two or three weeks elapsed before the members of the entoccelic cycle 

 began to appear. These were situated central to the first cycle, and in most cases the six appeared 

 togetlier, but a few exhibited a successive order, though of no regular character. For a long 

 time the newer tentacles nnuained smaller than the older, the usual entacmffious order being 

 thus reversed. 



On the completion of the prototentacular stage, the relationships of the tentacles and 

 mesenteries are as follows: The twelve primary mesenteries only have appeared, eight of which 

 are t'omplete and four incomplete, and the tentacles are outgrowths from the twelve mesenterial 

 chamlters, one from each. The six larger tentacles constituting the inner cycle are situated over 

 the six entocieles, and the six smaller tentacles of the outer cycle over the six exocccles. 



The rule that the tentacular sequence is associated with the stage reached in the mesenterial 

 development was first demonstrated by Lacaze-Duthiers (1S72) in the larvic of Actinia equina. 

 The i)rimary mesenteries in this species were found to appear in bilaterial pairs, according to a 

 regular sequence, and the tentacles conformed to this. Thus from the dorsal chamber, the 

 larger of the two produc^ed on the appearance of the first pair of mesenteries, appeared a 

 large tentacle, and from the ventral or smaller chamljer a smaller tentacle, both in the axial 

 plane. As the later pairs of mesenteries arose and chambers were formed, corresponding 

 tentacles appeared in a bilaterial manner until the twelve were estal>Iished. The primary 

 tentacles in ^1. equina retained the bilateral symmetry for some time, but ultimately this was 

 succeeded by the adult condition, in which the tentacles in any cycle are equal in size. 



All corals so far investigated, however, are provided with twelve fully established 

 mesenteries (eight comj^lete and four incomplete) and mesenterial chambers before the ten- 

 tacles begin to make their appearance. Hence, there is rarely any successive development in 

 their tentacular outgrowths, but the members of one or both cycles arise simultaneously — one 

 from each chamber. Where in Actinian larva>. less than twelve mesenteries are present, the 

 number of tentacles shows a corresponding diminution. Thus in larvae of Lehrunia mmUigens 

 only eight of the primary mesenteries were developed at the time of fixation, and but eight 

 tentacles appeared— in this case four large and four small. For nearly a week no increase of 

 mesenteries took place, and the tentacles, though modifying their comparative size, remained of 

 the same number. Some of the Actinian larvaj studied bv Lacaze-Duthiers also showed only 

 eight tentacles for some time, and Faurot (1S'.».J) has obtained similar results. 



A few observations have been made upon the appearance of the tentacles in budding polyps. 

 In the earliest stages determinable in buds of Poritts and Madrejwra., only six minute protuber- 

 ances can be distinguished, two median and four lateral, difl'ering somewhat in size. Older buds 

 with eight or ten tentacles may also be found. Such instances merely suflice to indicate that the 

 prototentacles of the bud do not arise simultaneously in the two genera mentioned, but in 

 median and then in successive bilateral pairs. The buds, however, arc .so minute as not to 

 permit of more detailed examination in the living expanded state, and scarcely anything can be 

 a.scertained from preserved colonies. 



The bilateral condition of the tentacles in the adult Madrepora and Porites, already referred 

 to, is full of suggestiveness from what is known of the tentacular development in the Actiniaria. 

 Lacaze-Duthiers (1S72) has shown that in Actinia equina an axial tentacle first appears, and that 

 for a long time this remains larger than the others, which arise in successive bilateral pairs." His 



"Dr. A. Appellof (1900, p. 79) doubts the accuracy of Lacaze-Duthiers' account and figures of the development of 



the tentiwles in Actinia equina, wliich have been accepted ahnost as classic. Aiuonjr hniidreds of larv;e of this species 

 investigated by him, Appellc'if has never met with tlie succession and proportional size of the tentai-les indicated by 



