74 SUMMARY OF CURRENT RESEARCHES RELATING TO 



mouth are the tentacles. The body-wall consists of ectoderm, meso- 

 derm, and endoderm. The first and last are composed of epithelial 

 cells, not separated by any intermediate substance. The mesoderm, 

 on the contrary, always consists of a continuous plate of hyaline sub- 

 stance, in which cells and groups of cells are very irregularly deposited. 

 From it there proceed to the digestive tube a number of radially 

 arranged, lamelliform processes, which are invested on either side by 

 endoderm, and with a varying amount of muscular fibres. The 

 number of these partitions is often constant for a whole group of 

 corals. 



The skeleton, under which term all the hard parts may be 

 included, consists either of numerous small particles, separated by 

 soft substances, or of larger, connected pieces, which may belong to a 

 single animal, or to a whole colony. The relative proportion of 

 organic to inorganic substances varies greatly, and we have all stages 

 between skeletons in which there is but a minimum of inorganic sub- 

 stances (horny skeletons), and those in which the inorganic materials 

 are superabundant (calcareous skeletons). The simplest forms of the 

 latter are the isolated and often microscopic spicules, which are found 

 in most Alcyonaria and in Polythoa. They are always found in the 

 mesoderm, rarely project into the ectoderm, and never into the endo- 

 derm. Occasionally simple, they are often very complicated in struc- 

 ture. Microscopic examination, after decalcification, shows that they 

 are formed of concentric layers of more or less horny intermediate 

 substance, and of calcareous crystals ; the layers of the former are very 

 thin, and can only be found in their natural positions with the greatest 

 care. In Gorgonia or Clavularia (the only two forms which have as 

 yet been examined with regard to this point), it has been found that 

 the spicules always arise in cells, which are always primitively ecto- 

 dermal, and afterwards pass more or less deeply into the mesoderm. 

 The spicules are at first smooth, often triangular, needles, which are at 

 first perhaps hollow, and only take on their definite form by the 

 gradual deposition of new layers. The nucleus of the mother-cell 

 is long persistent, but the protoplasm becomes a very thin layer. 



The larger calcareous masses, which are found in the axes of the 

 colonies of such furms as Corallium or Melithcea, arise from separate 

 spicules, which become connected together by the deposition of fresh 

 calcareous substance ; and that, without the sj)icules themselves 

 becoming altered in character. With this may be associated the 

 arrangements seen in Tubipora, where the separate spicules may be 

 seen gradually passing into a connected lamella. 



We come next to the Madreporaria, and here, if we take a separate 

 polyp, we may distinguish a sclerobase from the septa, which are con- 

 nected with it, and may be regarded as direct continuations of it ; 

 thirdly, there is a theca, which holds the same relation to the 

 sclerobase and the body-wall, as do the septa, with the peripheral 

 ends of which last it is fused ; lastly, there is the exotheca, which is 

 very frequently wanting, and which is nothing more than a continua- 

 tion of the sclerobase. This skeleton consists of crystalline spheroids, 

 which are cither directly connected with one another by means of their 



