102 



RADIATA. 



Obdee II,— HELIANTHOIDA. 



This order derives its designation from tlie resemblance borne by the polypes it includes to a sun-flower or 

 other composite blossom. The common Actinia may be taken as its type ; and aU 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 the general integument, so as to divide the intervening space into numerous chambers. The stomach is closed 

 at the bottom, as are also the surrounding chambers ; and this is equally 

 the case in the compound species as in the solitary. 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 tentacula, which 

 are provided with orifices at the extremity, that can be opened or closed by 

 the animal. Water is sometimes taicen in by these orifices, so as to distend 

 the radiating chambers and the tentacula ; and is then ejected with consi- 

 derable force tlirough the same apertures. There is reason to believe that 

 this is a respiratory process ; the whole interior of .the chambers, into which 

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



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

 Fio. 13 — Sectiok of Sea-Anemone c. ... ..ji ^x. ■ tj^ ,., .. 



o, cavity of stomach, b surrounding ctiambcrs. form plaited masses, attached along the inner border of some of the vertical 



leaflets which do not extend as far as the stomach. The ova appear to be developed in the substance of these 

 masses, and to escape, by the rupture of the membranous envelope of the ovarium, into the inter«eptal 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- 

 quired a stomach, mouth, and tentacula of its own. Young Actinice in this condition seem to be discharged, 

 not by the tentacular orifices, which are too minute to give tliem 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 tubuli, which are believed to be the male 

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



The Actinice proper do not usually increase by gemmation ; but this mode of increase has 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 Actinlv, In numerous other species of the order we meet with some form 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 Actinise 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 with 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 Polygastrio Infusoria (page 708). The polypes of these compound masses are connected by 

 a sort of gelatinous flesh ; but this would not seem to have the same degree of organization as that of the Alcyonian 

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

 means of a system of anastomising canals, as there is in the group next to be described. 



All the Corals which are distinguished as lamelUform, are formed by Helianthoid polypes ; deriving their cha- 

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



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 CaryophyUia or Fungia, it is 



marked on its upper surface by a single 



series of these plates (Fig. 14), strongly 



resembUng the gills of the mushroom ; 



but if the coral have been the axis of 



a compound mass, the radiating la- 

 mellae will be seen in every one of the 



individual polype-cells (Fig. 15), which 



are sometimes very numerous and mi- 

 nute, especially in the Madreporidffi. 

 Fig. 11.— CartcTphyllia. These cells are not by any means con- 



stantly circular ; but stiU the laminated plates project inwards fixim 

 their circumference towards a common centre. Sometimes a number of 

 cells unite into a groove or furrow; as in the 3fcandrina or brain Fio. is.— Mass of AfsTR.KA Viridis; o, o, m- 



,,,,., .., .. J. X • J 1 , panded Polypes; b. b. Polypes withdrawn into their 



Stone coral. In all these cases, the stony structure is produced by c«lU; c,Btony mass uncoverei by flesh. 

 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 undergoes a corresponding extension above. The stony axis, and its 

 lamellated cells, are thus really parts of the animal structure, and grow instead of being buHt up by the agency 

 of the coral polypes. The portion which has undergone consolidation, liowever, although continuous with the 



