124 



ZOOLOGY 



SECT. 



nv.c 



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 triggcr-liai-r (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 

 polype the cells are very large 

 and have the power of sending 

 out pseudopods at their free 

 ends (Fig. 84), 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 

 tiagella 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 inanu- 

 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. 84 and 86) 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 gonangium 



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



bell or umbrella (Fig. 83, B D) by which the zooid was originally 



attached to the blastostyle is distinguished as the ex-umbrella, the 



FIG. 86. Tentacle of Eucopella. The 

 lower part of the figure shows 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 ; ect. 

 ectoderm ; end. endoderm ; m.f. muscle- 

 fibres ; ntc. nematocyst ; nu. nucleus ; 

 nv.c. nerve-cell. (After von Lendeufeld.) 



