ECHINODERMATA 



429 



livisions of Echinoderms, which apparently seem to be so 

 5lear, the opinions of authors are nevertheless at variance 

 [comp. Semon, No. 55). 



[Through Ludwig's (No. XV.) recent investigations of Cucumaria 

 planci, these conditions have been satisfactorily elucidated. The five 

 ^invaginations of the water-vascular ring first formed really produce 

 I the radial vessels. They are, it is true, at first directed forward, but 

 [isoon bend backward, thus marking the radii. The tentacular vessels 

 do not arise directly from the ring-canal, but branch off from the radial 

 canals ; however, every radial canal does not give rise to a tentacular 

 vessel, the latter being distributed unsymmetrically to three radial 

 canals only. — K.] 



The internal condi- 

 tions are more evident 

 in Fig. 207, a longi- 

 tudinal section of the 

 pupal stage of Cucu- 

 maria (after Selenka). 

 The tentacular evagi- 

 nations are seen coming 

 off from the water- 

 vascular ring forwards, 

 and posteriorly those of 

 the radial vessels. The 

 Polian vesicles also take 

 their origin as evagina- 

 tions of the water-vas- 

 cular ring. In the stage 

 under consideration the 

 ring is still in connec- 

 tion with the outside 

 world by means of the 

 stone canal and the 

 dorsal pore. This con- 

 nection is afterwards 

 broken, since a cluster 

 of mesenchyma cells 

 subsequently applies 

 itself to the stone canal 



Fig. 207.— Longitudinal section of a larva of 

 Cucumaria doliolum, somewhat diagrammatic 

 (after Sklbnka). A, anus; Am, ambulacral 

 (radial) vessels ; D, intestine ; E, enterocoeles ; 

 F, feet; M, mouth; P, dorsal pore, leading 

 through the stone canal to the water-vascular 

 ring ; T, tentacular vessels ; Wr, water-vascular 

 ring. 



