702 



KADIATA. 



OitDEii II.—HELIANTIIOIDA. 



Thi'! order derives its designation from the resemblance borne by the polypes it includes to a sun-flower or 

 other composite blossom. The commoa Actinia may be taken as its type ; and all the animals which it includes 

 are constructed nearly upon the same model. The body is composed of a stomach possessing walls of its own, 

 and suspended by vertical partitions which pass in a radiating direction between the outer surface of the stomach 

 and thegeneral mtogument, so as to divide tlie intervening space into numerous chambers. The stomach is closed 

 at the bottom, as are also the surroundiog chambers; and tliis is equally 

 the case in the compound species as in the soUtary. The radiating parti- 

 tions have openings, by which the chambers communicate with each other ■ 

 and there is also a free passage from them into the hollow tcntacula, which 

 aiT iirovided with orifices at the extremity, that can be opened or closed by 

 tlif animal. Water is sometimes taken in by these orifices, so as to distend 

 tin.' radiating chambers and the tcntacula ; and is then ejected vvith consi- 

 d<.-rable force through the same apertures. There is reason to believe that 

 this is a respiratory process ; the whole interior of the chambers, ii]to which 

 the water is received, being covered with vibratile cilia. These chambers, 



^ ,„ ^ however, are specially intended for the development of the ova. The ovaries 



Fir.. 13.— SF.cTio>r of >5r v-Anemonk ; > i- ^ _ ->■ 



■'. .v.iity of stnmach, b surroundinE rhunihcrs. form plaited masses, attached aloQg the inner border of some of the vertical 

 leaflets which do not extend as far as the stotnach. The ova appear to be developed in the substance of the--e 

 masses, and to escape, by the rupture of the membranous envelope of the ovarium, into the interseptal spaces. 

 The embryo is sometimes discharged through the tentacular orifices, as a mere "gemmule;" but it is not 

 unfrequently retained within the body of the parent until it has undergone a further development, and ac- 

 q'lired a stomach, mouth, and tentacula of its own. Young Actinia: in this condition seem to be discharged, 

 not by the tentacular orifices, which are too minute to give them passage, but by the mouth ; although the 

 manner in which they pass from the ovarial chambers into the stomach is yet an unsolved mystery. Besides the 

 ovaria, these radiating chambers contain numerous long convoluted tubuh, ^vhicli are bL-lieved to be the male 

 organs. According to some, however, the sexes are distinct. 



The Actiinui proper do not usually increase by gemmation ; but this mode of increase hns been observed by Sir 

 J. G. Dalyell in one species, from the expanded base of which small portions occasionally detach themselves, 

 which subsequently become perfect AcUn%a\ In numerous other species of the order we meet with some fonn of 

 gemmiparous production, which gives rise to compound structures, resembling those of the other polypes, but 

 usually much more massive. Thus in the Zoanthus, we find animals that agree with the Actiniae in their general 

 organization, springing from a common base, which is sometimes broad and flat, but more commonly a sort of 

 creeping stem. In the arborescent species -n-itli a stony axis, however, the multiplication of the individual po- 

 lypes of the compound mass seems to take place by the division of the bodies of those already existing, very much 

 after the manner of the Polygastric Infusoria {page 708). The polypes of these compound masses ai-e connected by 

 a '^'jrt of gelatinous flesh; but this would not seem to have the same degree of organization as that of the Akyonian 

 X'ulypi; and there is no communication established between the digestive cavities of the individual polypes, by 

 means of a sy!:^tem of anastomising canals, a-^ there is in the group next to be deseribud. 



AU the Corals which are dtstinguishijd as himdUforra, are formed by Helianthoid pijlypes ; deriving thuir cha- 

 racter from the depusit of stony matter, not merely in the bases of the animals, and in the substance of tiie 



gelatinous flesh that connects them, ^ _ 



but also in the radiating partitions 



around the stomach. If the stony mass 



be the product of a single animal, as 



in the Caryophyllia or Fungia, it is 



marked on its upper surface bya single 



Series uf these plates {Fig. 14), strongly 



resembling the gills of the mushruum ; 



hut if the coral have been the axis of 



a comi'ound mass, the radiating la- 

 mella; will be Seen in every one of the 



individual polype-cells (Fig, 15), which 



are sometimes very numerous and mi- 

 nute, especially in the Madreporidce. 



These cells are not by any means con- 

 stantly c !( Ill a , hilt stdl the laminated plates project inwards fi-om 

 their circumference towards a common centre. Sometimes anumber of 

 cells unite into a groove or furrow; as in t\\ii Meandrina or brain 

 stiMie coral. In all these eases, the stony structure is produced by 



the consolidation of the lower and older portion of the animal, by means of a deposit of carbonate of lime, 

 whilst the softer or membranous portion undo.rgoes a corresponding extension above. The stony axis, audits 

 lamellated ceUs, are thus really parts of the animal structure, and grow instead of being huilt vp by the agency 

 of the coral polypes. The portion which has imdcrgone comsolidation, huwcver, although continuous witli the 



Fig. 15-— Mas 



ASTR^EA VrRIDl? 



I'ulypcs withdrami 

 uncori.TRd liy Hi.'^h. 



into tliuir 



