12 REPORT ON THE PENNATULIDA. 



The tentacles and pinnules being, as before stated, prolongations of 

 the body-wall, consist, like it, of ectoderm, mesoderm, and endoderm ; 

 but the intimate structure and relative proportions of the three layers 

 differ very considerably from those we have found to obtain in the 

 body-wall. 



The ectoderm (Fig. 11 w) is the thickest of the three layers : in it 

 the outlines of the component cells are very difficult to make out, and 

 it is only in the most favourable specimens that this can be done with 

 any certainty. The individual cells are long, thin, and columnar, 

 ciliated at their free ends, and arranged in a single layer, each cell 

 extending through the whole thickness of the ectoderm : in the deeper 

 parts of the ectoderm, between the bases of these columnar cells, 

 smaller cells of a spherical or fusiform shape occur, but in no great 

 number. 



Imbedded in and between the ectoderm cells are very numerous 

 thread-cells or nematocysts (Fig. 11 z), the characteristic weapons of 

 the Calenterata. Each of these is a capsule of an elongated oval shape, 

 and about 0-0004 in. long, within which is contained a long spirally- 

 coiled hollow thread, visible in many of our specimens when 

 examined with sufficiently high powers (^th or ^t\\ in.) In the Sea- 

 auemones, and in the common fresh water Hydra, in which similar 

 thread-cells occur, any external irritation, such as contact with a 

 foreign body, causes the thread to be shot out from the capsule with 

 great force and I'apidity, penetrating the irritating body, and exercising 

 on it, if an animal, an instantaneous numbing or paralysing action.* 

 We had no opportunity of testing their action iu the living FunicuUna, 

 but there can be no doubt that it is the same as iu the anemones 

 and Hydra. 



In the tentacles of Fiuiiculina the thread-cells (i'ig. 11) are most 

 abundant close to the surface, where they are closely packed side by 

 side, with their outer ends just beneath the surface, and their long 

 axes perpendicular to it ; large numbers also occur in the deeper parts 

 of the ectoderm. In shape and mode of arrangement the thread-cells of 

 Funiculina agree very closely with those of Sagartla troglodytes, as 

 described and figured by Heider,t and with those of the anemones 

 generally, as described by the brothers Hertwig I ; the thread-cells of 

 Hydra are larger and much more globular in shape. 



The mesoderm, which is the thinnest layer of the three, consists 

 almost entirely of muscles ; a very powerful external layer of longi- 

 tudinal muscles, seen cut across in Fig. 11, and an inner less powerful 

 layer of circular muscles. By these muscles the movements of the 



* For a very complete aucl admirable account of these thread-cells and then* 

 mode of action in sea-anemones, vide Gosse: "British Sea-anemoues and Corals, 

 18C0," pp. xxix-xl. 



f Heider. Sagartia troglodytes. Sitzb. der K. Akad. der Wissensch. z. Wien. 

 Bd. Ixxv. 1877, pp. '2'Z-2i ; and Plates III., IV., and VII. 



t Oscar mid Uichard Hertwig : Uic Actinieii. 1879. 



