134 



ZOOLOGY 



the surface and becomes embedded in one of the large ectoderm 

 cells. At one point of its surface the cnidoblast is produced into 

 a delicate protoplasmic process, the cnidocil or trigger-hair (cnc) : 

 when this is touched — for instance by some small organism 

 brought into contact with the waving tentacle — the cnidoblast 

 undergoes a sudden contraction, and the pressure upon the stinging 

 capsule causes an instantaneous eversion of the thread (B), at the 

 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 

 poljrpe the cells are very largo 

 and have the power of sending 

 out pseudopods at their free 

 ends (Fig. 96), 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. 96 and 98) the 

 endoderm (end) consists of a 

 single row of short cylindrical cells, nearly cubical in longitudinal 

 section: their protoplasm is greatly vacuolated and their cell-walls 

 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 gonotheca 

 — yet remains to be considered. The convex outer surface of the 

 bell or umbrella (Fig. 95, B — D) by which the zooid was originally 

 attached to the blastostyle is distinguished as the cx-umhrella, the 



Fio. 98 — Tentacle of Eucope 



lower part of the figure snows the ex- 

 ternal surface, in 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 ; cct. 

 ectodermi ; end. endoderm ; m f. muscle- 

 fibres ; ntc. nematocyst ; nu. nucleus ; 

 tiv.c. nerve-cell. (After von Lendenfeld.) 



