XVI 



ECHINODEKMATA 



535 



In this form the first five lobes of the hydrocoele give rise to the 

 radial canals, and the primary buccal tentacles arise from their bases. 

 Now in Synapta the radial canals disappear in the adult, they are, in a 

 word, vestigial functionless structures ; but in Cucumaria they persist 

 and give rise to numerous paired tube feet. Therefore the mode of 

 their development in Synapta is secondary, the method in Cucumaria 

 typical and primary, and this typical and primary method is the 

 same as that which obtains in other groups of Echinoderms. 



calc 



Fni. 398. The fully developed Auricularia larva of Synapta digitafu, viewed from the 

 ventral surface and from the side. (After Bury. ) 



A, Auricularia viewed from the ventral surface. The darker regions of the longitudinal ciliated band 

 indicate the fragments into which it will break at the metamorphosis. B, Auricularia viewed from the 

 left side. Letters as in previous figure. In addition, I-V, the fragments of the ciliated band which 

 will give rise to the transverse ciliated bands of the pupa. OR, the fragments of the ciliated baud 

 which will give rise to the ring surrounding the mouth of the pupa. li-5j, the lobes of the hydrocoele 

 which will give rise to the lirst tive buccal tentacles. 1-4, lobes of the hydrocoele giving rise to radial 

 canals, re, anus ; a.e, left anterior coelom ; a.d.a, antero-dorsal arm; c-il.cul, adoral band of cilia; 7i//, 

 hydrocoele ; int, intestine ; int.d.n, intermediate dorsal arm ; l.nerv, larval nervous system of Semon ; 

 I'.c. pore-canal ; p.d.a, postero-dors.il arm ; p.l.a, postero-lateral arm ; p.n.n, post-oral arm ; p.o.c, peri- 

 oral coelom ; I'ir.o.n, prae-oral arm ; st.r, stone-canal. 





The mesenchyme in Synapta has by this time formed a- loose 

 reticulate tissue spanning the blastocoele. In the posters-lateral 

 process certain of the mesenchyme cells have given -j-ise to typical 

 calcareous structures which enable us to distinguish the larva of 

 Si/napta digitata from other kinds of Auricularia (ca/r, Fig. 398, B). 

 These calcareous bodies are wheels consisting of a central knob, 

 numerous spokes, and a toothed rim. The knob does not lie in the 

 same plane as the rim, but beneath it, so that the whole might be 

 compared to a bowl with perforated sides as well as to a wheel. 



