REPORT ON THE ACTINIARIA. 43 



the different stages by which the ectodermal muscles are transformed into mesodermal. 

 As far as I could observe the supporting substance rises on the surface of the disk in 

 numerous folds covered with muscular fibrdlse. 



The tentacles are placed in two alternating rows, those of the inner row being rather 

 shorter and weaker than those of the outer row, whose length in a contracted condition 

 was 1 "0-1 "3 cm. They are thick- walled at the base, and run out into a fine point, 

 without any terminal opening. They are all strongly bent inwards, and have a hook-like 

 shape, which is caused by the distribution of the muscles. In most Actiniae, as we know, 

 the muscles surround the tentacles uniformly, but in Paractis excavata they are crowded 

 together towards the adaxial side where they form a muscular pad, which I have never 

 found equalled in strength in any other Actinia. In the tranverse sections (PI. XL fig. 

 14) the muscular fibrillse lie close together, and the framework of connective tissue is com- 

 pletely hidden, and only becomes distinct by appropriate staining ; it forms a network 

 whose meshes are small near the supporting lamella, but large and longish towards the 

 epithelium, enclosing spaces lying perpendicular to the surface of the tentacle. The 

 surface of the tentacles was not well preserved, so that I could not determine whether 

 these spaces were completely closed, or whether they communicate here and there with 

 the epithelium, which appears to me more probable. 



Over one half the circumference of the tentacle the muscular layer is of uniform 

 thickness, but thins out over the remaining half into a delicate membrane, which 

 seemed to me to be wanting at the base of the tentacle, unless perhaps it had been 

 rubbed off. In spite of the varying strength of muscular layer, the thickness of the 

 tentacle wall is essentially the same all through in transverse section, as the connective 

 tissue substance becomes thinner in proportion as the muscular layer becomes 

 thicker. 



It is, however, only the lower third of the tentacle which comports itself in the 

 manner above described, a transverse section through the point presents an essentially 

 different figure. The muscular layer is weaker indeed but present on all sides, it merely 

 becomes a little smaller for a short space on the abaxial side than on other parts of the 

 transverse section. A series of transverse sections rising from the base to the point shows 

 all the transitions between the two extremes, and we can follow step by step the process 

 by which the muscular layer, which originally lies only on one side of the tentacle, 

 gradually surrounds it entirely. I have only figured three transverse sections of such 

 a series, of which one is taken at the base (fig. 13, c), the second (fig. 13, b) from the 

 middle, and the third (fig. 13, a) from the point. In aU of these the thickness of the 

 muscular layer is indicated by hatching. 



The oesophagus is very short, corresponding to the height of the animal ; it is furnished 

 with two oesophageal grooves, and eighteen longitudinal swellings. Six pairs of septa 

 of the first order, and six pairs of the second order, are inserted in the oesophagus, 



