POLYPI 



[ 622 ] 



POLYPODIACE^E. 



by which it communicates with the abdo- 

 minal ca.vity. The 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 : oy spontaneous division, which is 

 mostly longitudinal and rare ; the forma- 

 tion of gemmae 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 b and 16 b). In these, the gemmae, 

 which are developed within them, and 

 which are often called ova, gradually be- 

 come bell- or disk-shaped, and assume the 

 form of Acalephce-, 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 (Hydra), or separately in 

 different individuals and internal (Actinia, 



&c.). 



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. HYDROIDA, and 

 Hincks's ' Zoophytes.' 



POLYPODIA'CE^E. An order of 

 Ferns, divided into 13 families by the cha- 

 racters of the sporanges. 



Synopsis of the Families. 

 * Sori with an indusium (except Alsophila). 



CYATHEJE. Sporanges numerous, sori on 

 a raised receptacle, with a somewhat ob- 

 lique annulus. 



DICKSONIE^E. Sori globose, on the back 

 or apex of a vein ; indusium subglobose, 

 free, at length bursting irregularly, more 

 frequently cup-shaped, entire or with 2 

 lips. 



HYMENOPHYLLEJE. 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. Sori 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^, 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^E. 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 edirr 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. 



SCOLOPENDEIE.S;. Sori as in Aspleniese, 

 but the indusia arranged in pairs and open 

 towards each other. 



AspiDiE^E. Sori dorsal, subglobose, 

 rarely elliptical ; indusium superior, shaped 

 like the sorus, fixed either by the centre or 

 a sinus. 



