KEPORT ON THE PEXXATULIDA. '2,9 



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

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

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

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

 ciliated at their fi-ee ends, and arranged in a single layer, each cell 

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

 pai'ts 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 Calentercitn. 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 ^-th in.) In the Sea- 

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

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

 foreign bod}-, causes the thread to be shot out from the capsule with 

 great force and rapidity, 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 in the living FunicuUiia, 

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

 and II 1/(1 ra. 



In the tentacles of Funiculinii the thread-cells (Fig. 11) are most 

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

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

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

 of the ectoderm. In shape and mode of ai'rangement the thread-cells of 

 FunicuUiia agree very closely with those of Sacjartia troglodijtei;, as 

 described and figured by Heider.f and with those of the anemones 

 generally, as described by the brothers Hertwig J ; 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 

 tentacles in the living animal are effected, and also, in part, the 

 retraction of the tentacles when disturbed. 



The endoderm does not differ markedly from that of the body- 

 wall : it consists of a single layer of columnar cells, often swollen at 

 their inner ends. In the cavities of the tentacles, the size of which 

 varies much with the extent to which the tentacles are expanded, very 



* For a very complete and admu-able account of these thread cells and their 

 mode of action iu sea-anemoues, vide Gosse: "British Sea-auemoues and Corals, 

 1860," pp. xxix-xl. 



f Heider. Sagartia troglodytes. Sitzb. der K. Akad. dcr Wissousch. z. Wiou 

 13d. Ixxv. 1877, pp. •2-2-21 ; and Plates III.. IV., and VII. 

 : Oscar und Kichaid Hertwig : Die Actiuieu, 1879, 



