XVI 



ECHINODEKMATA 



551 



substratum by the secretion of the prae-oral pit. At first it lies 

 with its length parallel to the substratum, but it soon becomes 

 erected so as to stand at right angles to the substratum. The stomo- 

 daeum becomes completely closed from the exterior, and so resembles 

 the amniotic space of the Echinopluteus 

 larva; it rotates backwards along the 

 ventral surface till it comes to occupy 

 the posterior pole of the larva, the body 

 of which becomes differentiated into a 

 narrow anterior portion, the stalk, and 

 a posterior broader part, the cup or 

 calyx. This calyx becomes pentagonal 

 in section, and so the radii of the 

 Crinoid are marked out, and the lobes 

 of the hydrocoele soon come to corre- 

 spond with the sides of the pentagon. 



The fixation pit flattens out to 

 form a fixing disc, which becomes sup- 

 ported by a calcareous foot-plate. The 

 cilia are shed, and the cells which 

 formed the ciliated bands secrete a 

 cuticle and then retreat from the surface 

 to some extent, touching it only by 

 thin prolongations ; processes from the 

 mesenchyme cells extend up between 

 them and it soon becomes absolutely 

 impossible to discriminate between 

 ectoderm and mesenchyme. The same 

 fate befalls the ectoderm cells forming 

 the intermediate areas, indeed it some- 

 times happens to them before fixation. 

 The ectoderm lining the stomodaeum, 

 which has now become the closed 

 vestibule, however, undergoes none of 

 these changes. Where it covers the 

 hydrocoele and the tentacles it is a 

 thick syncytium, elsewhere it is a thin 

 layer of flattened cells. 



The number of tentacles becomes 



axt 



Fig. 407. — Fixed larva of Antedon 

 rosacea, three and a half days after 

 hatching, viewed from the side — 

 decalcified. (After Seeliger. ) 



ax, axial organ containing the genital 

 stolon ; aic.f, axial band of fibres surround- 

 ing the chambered organ; an.t, azygous 

 primary tentacle of the hydrocoele ; int, 

 rudiment of the intestine ; int.t, inter- 

 j . . i. £ 1, j-u „••,„„■.,„»,«« radial tentacles; oes, oesophagus; p.t, 



raised to twenty-five by the appearance ^^.^^^ tentacles; sc^, rudiment of sac- 

 of another pair in each radius, situated cuius ; r.p.e, right posterior coeiom ; st, 

 below those already formed and ap- stom^^h ; »«™. i"™' stomodaeum now 



, , ..•',. . 1 (. ,i. become the vestibule. 



parently springing directly irom the 



hydrocoele. The first pair of tentacles also come to spring directly 

 from the hydrocoele ring, by the absorption into this ring of the base 

 of the primary tentacle of which they were outgrowths. All these 

 tentacles become long and protrude into the vestibule, but the last 

 formed are shorter and not so extensile as the first formed. All the 



