242 DESCRIPTIONS OF PREPARATIONS. 



wall and base, and contains fibrils and stellate or fusiform cells. The supporting 

 lamina of the mesenteries is continuous with that of the wall. The endoderm 

 is unilaminar, its cells are flagellate, and the majority possess basal muscle-fibres. 

 These fibres are disposed in a circular direction on the wall, peristome, stomodaeum, 

 and the base where they are sometimes wanting. They are longitudinally disposed 

 on the intra-septal surfaces of the mesenteries ; transversely on the inter-septal. 

 The ridge forming the tentacular muscle is produced by folds of the supporting 

 lamina on which the muscle-fibres lie. A parieto-basilar muscle running obliquely 

 from the side of the wall to the base is developed similarly from the transverse 

 muscle layer on the inter-septal surfaces. These two muscles, tentacular and 

 parieto-basilar, are reversed in position on the surfaces of the directive septa. 

 The endoderm also possesses gland cells and sense-cells. Ganglion cells with 

 nerve fibrils have been found in it, but not so numerously as in the ectoderm of 

 the peristome. Sense-cells and nerve-fibres are found most plentifully in the 

 median process of the mesenterial filaments and the acontia. 



Ectodermal muscle-cells occur in some forms on the wall and stomodaeum, 

 ganglion cells on the wall and base. 



The surface of the body, as in Tealia, is sometimes roughened by tubercles, 

 which are local elevations of the supporting lamina. The coloured ' marginal 

 spherules,' which are found outside the tentacular circle in Actinia mesembryan- 

 themum^ &c., are evaginations of the whole body wall. Their ectoderm is 

 thickened, and contains many thread-cells. Gland cells are especially numerous 

 near and round the base of these bodies, which must be regarded as batteries of 

 threao^cells. 



Unicellular Algae, the well-known ' yellow cells,' live in the endoderm cells of 

 many Actinians, but not in Tealia. They are never found in those Actinians which 

 have a red colour, only in those which are colourless, e.g. Aiptasia diaphana, &c., 

 or in the colourless parts of a coloured Actinian, as in the tentacles of Ceriactis 

 aurantiaca. The association between the two organisms is known as Symbiosis. 



The term ' Symbiosis ' was first used by the botanist, De Bary, to denote the 

 association together inter se of different animals, or of different plants without 

 reference to the character of the association. The term has also been used in the 

 same manner by Hertwig. It is now very generally restricted to mean the asso- 

 ciation together of two different organisms which are physiologically the complements 

 of one another. Such an association may be formed between two plants, or 

 between a plant and an animal. 



Among plants the group of Lichens consists of an assemblage of forms in 

 which a fungus grows in the thallus of an alga. Both fungus and alga are capable, 

 at least in some instances, of living and growing independently of one another. 

 A single alga may act as host to a large number of different fungi ; and vice 

 versa, a single fungus may affect a number of different algae. In some instances 

 the alga grows more vigorously when surrounded by the hyphae of the fungus ; 

 in others the hyphae have been observed to penetrate the algal cells and destroy 

 their contents after the manner of a parasite. Another remarkable instance of 

 symbiosis among plants is the growth of a fungoid mycelium round the rootlets 

 of certain Phanerogams Orchidaceae ; Monotropa; the Cupuliferae, e.g. oak, 

 hazel, beech ; Abietinae, Salicaceae, and Betulaceae, e. g. alder, birch. 



