MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 513 



given an account of the longitudinal lission in the common Actinian, Metridhim marginatum 

 Milne Edwards, and .shows that the asexualh' formed polyps do not reproduce all the featuj-es 

 characteristic of the sexually produced individuals. In some of the specimens examined, Parker 

 found that fission of the stomodival tube was still incomplete, the oi'gan being Y-^haped, a single 

 inner end opening into the gastro-vascular cavity, and the two outer ends opening each through 

 a distinct mouth on a single disk. Generally in the fission specimens each mouth was mono- 

 gl3'phic (provided with only one gonidial groove), instead of diglyphic, as in normal forms; and 

 with the monogh'phic condition was associated only one pair of directives. No evidence was 

 forthcoming as to the formation of new siphonoglyps or new directives in fission polyps, while 

 there were practically twice as manj' non-directive mesenteries in double specimens as in single 

 ones. In any given case the assumed plane of division passed through either two primary 

 exocoeles or two entocojles, nev-er a primary entoccele on one side and a primary exocoele on the 

 other. The production of regular hexamerous diglyphic specimens 1)y non-sexual methods was 

 not observed; such specimens were found to number about one-fifth of the total collected, and 

 are with good reason assumed to be the products of sexual reproduction. 



The West Indian stichodactylinous anemones, Actinotryx sanctl-Tlioinx Duch. and Mich., 

 and Rimvdea fiorida Duch. and Mich., also reproduce by discal fission, and f requentlj' more than 

 one oral aperture is pi-esent within a single tentacular zone. In the latter species as many as 

 seven mouths have Ijcen found on a single disk, thus recalling such a coral form as Maniclna. 

 Professor McMurrich's (1SS9«) anatomical studies of these two species, and also mine (1900), have 

 shown that the mesenteries in both species are irregularly' arranged, and in some polyps no 

 directives occur, while in others only one pair is present. 



The results on poh'pal fission in corals may be thus summarized: 



1. The larval polyps of fissiparous species develop for a time like other hexamerous species. 

 Before the introduction of fission the mesenteries are regularly arranged in two or more alter- 

 nating hexameral cycles, and two jsairs of directives are present. 



2. The first fission plane passes through the entoccele of two lateral mesenterial pairs, 

 approximately at right angles to the directive plane and longer oral axis, and divides the 

 stomodieum and the mesenteries attached to it into practically equal halves, so that onlj- one 

 pair of directives is inserted on each stomodseal tube. 



3. For a time the jjroducts of simple fission continue their development according to the 

 regular cyclic plan, ])ut before long fission is repeated, and each stomodteum and the mesenteries 

 associated with it may be again divided into equal halves, or one part may be larger than the 

 other. In probably every division the fission plane is included within two opposite entoco?les. 

 At an eai'ly stage in the development of fission polyps growth may proceed more rapidly at one 

 region than at another, and thus introduce irregularities in the cyclic plan. 



4. Bej'ond the two prinnuy pairs no new directive mesenteries are ever introduced, so that in 

 any fissiparous colon}', however large, only the protocnemic directives occur, situated widely 

 apart, at what ma}' be regarded as the two morphological extremities of the colony. 



With such results before one, it is clear that care must be exercised in attemjiting to establish 

 relationships from the absence of one or both pairs of directives in mature polyj)s of both corals 

 and Actinians. The history of the individual polyp must be taken into account before such an 

 occurrence can be regarded as a specific peculiarity. As a general rule, one would be justified 

 in assuming, asexual reproduction l)y fission for poh'2:)s with onl}' one or no directives, especiall}^ 

 if accompanied by irregularities in mesenterial growth. 



FISSION IN PORITES. 



Polyps displaying a bioral disk arc \erv rare on colonies of Parltes. After an examination 

 of scores of living colonies of all the West Indian species, onl}' a single example exhibiting this 

 condition has been found, although polyps showing an increase of tentadi^s beyond the usual 

 twelve are bj- no means scarce; from these all stages in the development of the mesenteries 

 beyond the primary six paii's are readily secured. These proportions present a marked contrast 



