INTRODUCTION. 25 



three dimensions, those of raecliiun size being the commonest ; also, of very simple cells, 

 either irregular in shape, or oblong or pyriform ; and of small rounded and transparent 

 globules, which form the innermost layer. 



" There are no cells in the external tegument which produce the colour of the 

 polype. The white warts which project ou the surface are made up of a mass of large, 

 transparent and elongated vesicles. 



" The nematocysts, which form the most important part of the integument of the 

 tentacules, are slender and cylindrical, one of their extremities being smaller than the other .^ 

 They contain a thread regularly rolled up as a spiral, and which near the large end termi- 

 nates in a straight and central portion. The thread when unrolled is about two tenths of 

 a millimetre in length. The nematocysts are perpendicvilar to the tentacular surface, 

 and their large end is the most external ; the internal thread makes its exit by this 

 extremity. 



" The terminal bulb" of the tentacules is almost entirely composed of these filiferous 

 capsules ; there are two other kinds in it unlike those just described, some larger and 

 stouter, and others much narrower and more slender. The first are elliptical, slightly 

 attenuated at one of their ends, and they contain a thread rolled into a slack spiral. This 

 thread shoots out from the small end of the cell. The remaining nematocysts do not 

 appear to have a proper cell-wall ; they are cylindrical, slightly smaller at both ends, and 

 very slender; they are formed by a filament very closely rolled into a dense spiral, which 

 unrolls itself like the wires used in some elastic clothing.^ 



" The -structure of the skin is the same over the whole surface of the polype. The 

 nematocysts of the second size are the most common. A certain number of those of the 

 largest size are found in the stomacho-buccal rim. The cilia are very distinct at this spot, 

 and around the disc also, although they are very delicate ; they are rare and feeble on , 

 other parts of the polype ; they are very indistinct on the tentacules, and are wanting on 

 the bulb. 



" 2. The middle or muscular layer is formed by transverse and vertical fibres which 

 are excessively slender and sparely distributed. Very thin oblique muscular fibres may 

 be seen at the bases of the tentacules. 



" 3. The internal membrane is formed by a layer of transparent cells tolerably adherent 

 to each other, and by a layer of colour-bearing globules which are spherical or slightly 

 oval in shape. > 



" It is these cells which give the colour to the polype ; they are filled with irregular- 

 shaped grains, of a bright brown colour ; they themselves are secreted in certain trans- 

 parent vesicles, and present the greatest resemblance both in shape, colour, and structure 

 to the globules which float free in the tentacular cavities of young sea-anemones. It is , 

 probable they have a corresponding function in their early age. Near the top of the : 



1 Plate II, fig. 1. 2 Plate II, fig. 3. » Plate II, fig. 6. 



4 



