IV 



PHYLUM CCELENTERATA 



135 



-nu 



nv.c 



base of which are minute barbs. The threads are poisonous, and 

 exert a numbing effect on the animals upon which Obelia preys. 



The endoderm also has the general character of a columnar 

 epithelium. In the body of the polype the cells are very large 

 and have the power of sending out pseudopods at their free 

 ends (Fig. 97), which apparently seize and ingest minute portions 

 of the partly-digested food. As in many Protozoa, the pseudo- 

 pods may be drawn in and long flagella protruded, the contrac- 

 tion of which causes a constant 

 movement of the food particles 

 in the enteron. Amongst these 

 large cells are narrow cells with 

 very granular protoplasm : they 

 are gland-cells, and secrete a 

 digestive juice. In the manu- 

 brium a layer of endodermal 

 muscle-fibres has been described 

 taking a transverse direction, 

 and so serving to antagonise 

 the longitudinal muscles and 

 contract the cavity. In the 

 tentacles (Figs. 97 and 99) the 

 endoderm (end) consists of a 

 single row of short cylindrical 

 cells, nearly cubical in longitu- 

 dinal section : their protoplasm 

 is greatly vacuolated and their 

 cell-walls are so thick that they 

 may be considered as forming 

 a sort of internal skeleton to the 

 tentacles. 



The structure of the medusae 

 formed, as we have seen, by the 

 development of medusa-buds 

 liberated from a ruptured gono- 

 theca yet remains to be con- 

 sidered. The convex outer sur- 

 face of the bell or umbrella (Fig. 96, B D) by which the zooid was 

 originally attached to the blastostyle is distinguished as the ex- 

 umbrella, the concave inner surface as the sub-umbrella. From the 

 centre of the sub-umbrella proceeds the manubrium (mnb), at the free 

 end of whi^h is the four-sided mouth (mth). Very commonly, as the 

 medusa swims the umbrella becomes turned inside out, the sub- 

 umbrella then forming the convex surface and the manubrium 

 springing from its apex (Fig. 96, C, and Fig. 100, A). 



The mouth (Figs. 96, 97, 100, and 101, mth) leads into an enteric 

 cavity which occupies the whole interior of the manubrium, and 



ntc 



FIG. 99. Tentacle of Eucopella. The 

 lower part of the figure shows the ex- 

 ternal surface, hi the middle part the 

 ectoderm is removed and the muscular 

 and nervous layer exposed, in the 

 upper part these latter are removed so 

 as to show the core of endoderm cells. 

 ect. ectoderm ; end. endoderm ; m.f. 

 muscle-fibres ; ntc. nematocyst ; nu. 

 nucleus ; nv.c. nerve-cell. (After von 

 Lendenfeld.) 



