POLYPI. 



[ 622 ] 



POLYPODIACE.E. 



by which it communicates with the abdo- 

 minal cavity. Tlie gastric cavity is covered 

 with ciliated epithelium, which is continued 

 through the gastric apertures into the ab- 

 dominal cavity, and here not only covers 

 the outer surface of the stomach and the 

 septa, but also the inner surface of the ab- 

 dominal walls, the cavities of the arms, and 

 the canals of the polypidom. 



The walls of the stomach are variously 

 coloured, white, yellow, or brown, from the 

 presence of aggregations of pigment-cells 

 (liver cells), which probably perform the 

 function of a liver, as there is no glandular 

 appendage corresponding to a liver pre- 

 sent. 



A peculiar circulation takes place in most 

 polypes, by the to-and-fro motion of a 

 nearly transparent liquid containing minute 

 colourless corpuscles, in the abdominal 

 cavity. In the colonial polypes, this circu- 

 lation continues through the canals which 

 traverse the polypidoms, from one abdo- 

 minal cavity to the other. 



The propagation of the polypes takes 

 place : by spontaneous division, which is 

 mostly longitudinal and rare ; the forma- 

 tion of gemnue or buds, which is very com- 

 mon, the individuals either separating or 

 remaining attached ; and by the formation 

 of ova. Most of the gemmae become deve- 

 loped into ordinary polype-cells, and so 

 produce the growth of the compound orga- 

 nism. But in many polypes, some of them 

 grow into large cells, of different form from 

 the common polype-cells, constituting the 

 ovarian vesicles, or capsules (PI. 41. figs. 

 14 i and IGi). In these, the gemmae, 

 which are developed within them, and 

 Avhich are often called o^a, gradually be- 

 come bell- or disk-shaped, and assume the 

 form of Acalephcp. ; and on escaping from 

 the vesicles, swim about freely, subsequently 

 either becoming directly developed into 

 new polypes, or acquiring sexual organs 

 and producing ova. 



In other (simple) polypes, distinct sper- 

 matic and ovi-sacs occur in the parent 

 animals; either together in the same ani- 

 mal, and external {Hyiira), or separately in 

 different individuals and internal {Actinia, 

 etc.). 



The ovum-embryo of the polypes is 

 usually more or less elongate-oval, coated 

 with cilia, and moves about on its long 

 axis like an infusorium. After a short 

 time, it fixes itself to some object, the cilia 

 disappear, and the tentacles of the polype 



are protruded ; as these polypes increase by 

 gemmation, new colonies are formed. 



The formation of coral reefs and islands 

 by the skeletons of polypes is well known. 



The new names of the Polype-structures 

 will be found in the Art. Hydeoida, and 

 Hincks's 'Zoophytes.' 



POLYPODIA'CE-'E. — An order of 

 Ferns, divided into 13 families by the cha- 

 racters of the sporanges. 



Si/no2)sis of the Families. 

 * 8ori tvith an indusium {except Alsophila). 



Cyathe.^. Sporanges numerous, sori on 

 a raised receptacle, with a somewhat ob- 

 lique annulus. 



DiCKS0NiE.i3. Sori globose, on the back 

 or apex of a vein ; indusium subglobose, 

 free, at length bursting iri'egularly, more 

 frequently cup-shaped, entire or with 2 

 lips. 



Hymenophylle.^. Sori terminal or 

 marginal from the apex of a vein ; recep- 

 tacle elongate, often filiform, and more or 

 less exserted, clothed with sessile orbicular 

 subpeltate compressed capsules, with a 

 transverse ring opening vertically. 



Davallieje. SoiT rounded, marginal or 

 submarginal, covered by a reniform or 

 rounded scaly indusium open at the apex, 

 fastened broadly at the base, and open at 

 the sides. 



LiNDSAYE.'E, . Sori in a line at or near 

 the end of the frond, inner valve of indu- 

 sium membranous, the outer formed of the 

 margin of the frond. 



Pteride^. Sori marginal, linear or ob- 

 long ; indusium of the same shape as the 

 sorus, formed of an altered and reflexed por- 

 tion of the frond, opening inwardly. 



Blechne,^. Sori linear or oblong, dor- 

 sal, parallel with the midrib and edge of 

 the segments, not close to the latter ; indu- 

 sium shaped like the sorus, superior, open- 

 ing towards the midrib. 



AsPLENiE^. Sori attached to the veins, 

 oblique or subparallel to the midrib, linear 

 or oblong. Indusium shaped like the sorus, 

 when single, opening towards the midrib, 

 sometimes double. 



ScoLOPENDRiE.E. Sori as in Aspleniere, 

 but the indusia arranged in pairs and open 

 towards each other. 



AspiDiEiE. Sori dorsal, subglobose, 

 rarely elliptical ; indusium superior, sliaped 

 like the sorus, fixed either by the centre or 

 a sinus. 



