^JrO 



ECHINODERMATA 449 



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'ocess of the body cavity. We shall revert to this part of 

 the body cavity and its derivatives later. 



An entirely clear insight into the structural conditions of the body 

 cavity, which are obviously difficult to follow, cannot be gained from the 

 authors' statements (Goette, Perkier, Barrois, Bury), since they do not 

 agree. The older statement of Goette, according to which the body 

 cavity also takes part in the formation of the vestibule, appears in 

 another light since the descriptions which Barrois and Bury give of 



Fig. 220.— Longitudinal section through the cup of an Antedon larva, the vesti- 

 bule of which is still closed (after Goette, from Balfour's Comparative Embryo- 

 logy), ae, depression of the vestibular epithelium to form the mouth (m); al, intestinal 

 canal; an, region of the anus; Ip, subambulacral body cavity; Ip', vestibule; m, 

 mouth; mt, transverse mesentery ; r, roof of the vestibule ; rp, visceral part of the 

 body cavity and its prolongation {rp') into the stalk of the larva ; t, tentacle ; tcr, 

 water-vascular ring. 



this process. According to these authors, the lining of the vestibule 

 is not mesodermal, but ectodermal. The penetration of an enterocoelic 

 diverticulum into the stem, observed by Goette, Perrier, and Bury, is 

 denied by Barrois. According to him, these axial structures arise rather 

 by an accumulation of mesenchyma cells. On the other hand, according 

 to Barrois, a process of the subambulacral body cavity penetrates axially 

 toward the stem. Bury's contention that, in addition to the right and 

 K. H. E. G G 



