554 



INVEETEBEATA 



CHAP. 



of which is supported by an oral plate, and the tentacles are protruded 

 between the valves. The epithelium lining the vestibule becomes 

 thin, except just over the hydrocoele ring where it becomes thick and 

 ciliated, and beneath this ring of thickened epithelium a ring of nerve 

 fibres makes its appearance. The high ciliated epithelium and the 

 nerve fibres are prolonged over the inner sides of the tentacles. 

 Sense-organs are scattered over the surface of these tentacles. Each 

 sense-organ consists of a conical prominence of elongated ectoderm 



oes 



ab.c. 



Fig. 409. Calyx of larva of about the same age as that represented in Fig. 408, decalcified 

 and cleared in order to show the internal structures. (After Ludwig. ) 



ax, axial organ containing the genital stolon ; ab.c, aboral coelom ; hy.r, hydrocoele ring ; m.p, primary 

 madreporic pore ; or.c, oral coelom ; or.v, oral valve ; st.c, stone-canal. 



cells. The tips of the cells forming the apex of the cone become 

 elongated so as to form a tactile rod. 



The wall separating the aboral coelom and the anterior coelom 

 thins out till it forms a flat membrane of epithelium, which is then 

 absorbed and the two cavities which it separated become merged 

 into one. 



In this manner we reach the condition found in the adult 

 Crinoid, where the stone-canal opens at one end into the general 

 coelom, and the pore-canal takes its origin independently from this. 



