REPORT ON THE ACTINIARIA. 5 



Among the histological elements of the Actinia we must finally mention the 

 muscle cells, nerve cells, and rei^roductive cells ; we shall merely discuss the two former 

 here from a general point of view. The muscles originate either from the ectoderm 

 or the endoderm, and usually continue to belong to both these epithelial layers. They 

 consist of flat, fusiform, muscular fibrillse, to one side of which the cell from which they 

 were originally produced is attached. This latter is usually at the same time an epithelial 

 cell, and with the fibre belonging to it represents an epithelio-muscular cell, or it is a 

 cell lying in the deeper layers of the epithelium, and no longer extending as far as the 

 surface, an epithelial cell, whose peripheral end has undergone retrograde formation, or 

 a subepithelial muscle cell. 



The principle of arrangement of the fibrillse is the same in both cases ; they are placed 

 on the borders of the epithelium and the mesodermal connective substance, and form 

 a thickly apposed simple layer, a muscular lamella. The muscles are not strengthened by 

 the deposition of new layers of fibres, but by the " pleating " of the single-layered lamellae. 

 The underlying connective substance also comes into play, supporting all the folds of the 

 muscular lamella by fine leaf-like processes (PL V. figs. 7-10 ; PL VI. figs. 4, 6). 



The pleating of the epithelial, or subepithelial muscular lamella, becomes in many 

 cases the starting-point for the development of a third form of the muscular fibres, the 

 " mesodermal " fibres. When the surfeces of the supporting substance, which borders a 

 muscular fold laterally, approach so that here and there they touch and become fused, 

 the connection of the lower part of the pleating with the epithelium is dissolved, and 

 it becomes completely enclosed in the mesoderm (PL VII. fig. 8). In this way are found 

 in transverse sections, circular figures, whose periphery is occupied by the divided 

 fibrillse, whilst the centre contains the muscular corpuscles belonging to it. The trans- 

 formation of the epithelial muscular elements into mesodermal can go so far that 

 considerable masses of muscles lie in the mesoderm (PL IV. figs. 5-8 ; PL VI. figs. 1-3, 5). 



In describing the muscles of the Actiniae we must, therefore, be careful to note 

 whether they are ectodermal, endodermal, or mesodermal, whether they extend simply 

 in a smooth lamella, or are disposed in folds ; as we shall see, they present in this way 

 many characteristics of systematic value. This cannot be said of the nervous system, which 

 I only go into here for the sake of completing my description. Nerve fibres and ganglion 

 cells are found, in thoroughly examined Actiniae, in nearly all the epithelial laminas, 

 where they form a layer between the bases of the epithelial cells. The layer is extremely 

 thin in the ectoderm of the pedal disk, and usually also in that of the wall, whilst 

 it is very strong in the ectoderm of the tentacles, of the oral disk and of the oesophagus. 

 Nervous elements are usually less frequent in the endoderm, and only produce visible 

 cords in the mesenteric filaments and acontia. We may lay down as a rule, that, where 

 muscular filaments are present, the layer of nervous filaments Lies over the former, 

 and is most easily found in that place. 



