559 



brings the edges of the teeth in contact, at the same time necessarily 

 inclining them towards the centre, and thus effectually closes the 

 disk ; the animal being then entirely covered and protected by the 

 investing sandy coat. It will be observed that the apparently "mar- 

 ginal teeth are in reality only parts of the wall of the column, and 

 that intervening triangular pieces are as it were excavated from the 

 integuments, leaving only the internal membrane and muscular bands. 

 The nature of this adventitious covering also deserves attention, being 

 the only character in which this polype at all resembles Dysidea 

 fragilis — the sponge with which it was formerly associated. It is 

 almost entirely composed of fine angular particles of siliceous sand, 

 brought in contact with the body and connecting membrane of the 

 polype by the action of the sea, and retained by, and incorporated in 

 the cuticle ; its extraneous character is evident from tbe occasional 

 presence of other matters mixed with the sand, but the latter sub- 

 stance is in most cases the only material employed. Similar grains 

 of sand abound in the sponge ; they are not confined, however, to the 

 exterior, but are scattered throughout the mass, and cover the inter- 

 lacing fibres in every direction. 



Independently of its different composition, this sandy coating in 

 Zoanthus cannot be regarded as at all analogous to the true corallum 

 of the Madrepores ; here it is the actual polype which is enclosed 

 in the hard covering, and this, when tested with nitric acid, shows no 

 trace of calcareous matter ; in the Madrepores, on the contrary, the 

 polype is as delicate and soft-bodied as any of the Actiniae, and when 

 expanded, rises above and clothes the upper portion of the corallum, 

 which is entirely secreted by the internal tissues of the animal, and 

 is composed essentially of carbonate of lime extracted from the sea- 

 water ; in fact, the hard parts constitute an external covering in the 

 one animal, and an internal skeleton in the other. 



In its explanate growth, or increase by budding from the base 

 only, Zoanthus strongly resembles Caryophyllacea, and by some 

 naturalists is associated with that tribe of Coralligenous Polypes ; but 

 many of its characters point to a nearer relationship to the Actinidce, 

 in which we sometimes find a similar deposition of extraneous matter 

 on the cuticle, although in a slighter degree and less persistent : the 

 smooth simple tentacula are also very unlike those of the Coral Po- 

 lypes, in which their surface is generally studded with little wart-like 

 prominences enclosing the thread-cells. "With our present scanty 

 knowledge of the Actinidce found in different parts of the world, and 

 the insufficient descriptions that we possess of most of the coral 

 animals, it is difficult, if not impossible to determine, the true position 

 of the Zoanthidce among the Helianthoid Polypes. An examination 

 of the tropical seas, in which they abound, and where they attain a 

 size considerably exceeding that of our British species, may lead to 

 the discovery of intermediate forms showing the true affinities of this 

 now isolated group ; but at present 1 am inclined to regard them as 

 representing the budding form of growth in the Non-coralligenous 

 Zoophytes, as the fissiparous mode of increase is exemplified in many 

 of the true Actiniae. 



