268 



PHYLUM ECHINODERMATA. 



Tube-feet and anibulacral papillae absent. Mouth and anus terminal. 

 Body cylindrical ; round the tentacular circlet it widens out into a thin 

 disc, the edge of which is produced into long rays ; 13 to 16 tentacles ; the 

 tentacular canals arise from the well-developed radial canals, and they 

 each give off at their base a canal (? ampulla) into the disc. Calcareous 

 ring absent. The stone-canal opens on the surface of the body. Longi- 

 tudinal muscles 

 simple ; retractors 

 absent. Transverse 

 muscles interrupted 

 in the radii. Re- 

 spiratory trees, cili- 

 ated cups and Cuvi- 

 erian organs absent. 

 Genital organs right 

 and left of the 

 dorsal mesentery. 

 They swim on the 

 surface of the sea 

 by means of their 

 disc. Pelagothuria 

 Ludwig. 



Fam. 5. Molpa- 

 diidae. Burrowers. 

 Feet and ambulac- 

 ral papillae absent. 

 Mouth terminal. 

 Hinder end of body 

 often reduced to a 

 tail-like appendage. 



Tentacles small, usually 15 in number, cylindrical or provided with 

 some small branches near the end ; tentacular ampullae present, pro- 

 jecting into body-cavity. Calcareous ring of 5 radial and 5 interradial 

 pieces. Stone-canal always single, often fastened to the body-wall. 

 Otolithic vesicles absent. Longitudinal muscles divided, retractors well 

 developed in Molpadia only. Respiratory trees present. Cuvierian 

 organs only in one species. Principally on mud or clay. Molpadia Cuv., 

 Eupyrgus Liitken, Haplodactyla Grube, Caudina Stimp., Trochostoma 

 D. and K., Ankyroderma D. and K., anchor shaped spicules in this genus. 

 Sub-order 2. DENDROCHIROTAE, with branched tentacles and re- 

 tractor muscles of the pharynx. Tentacular ampullae (not projecting), 

 and respiratory trees always present. 



Fam. Cucumariidae. With feet, rarely with ambulacral papillae. 

 Mouth usually dorsal or terminal. Anus often dorsal. Body cylindrical 

 or pentagonal or with a ventral sole. With 10-30 branched tentacles 

 often unequal in size, the two ventral being generally smaller than the 

 rest (Fig. 177) ; tentacular ampullae not distinct. Calcareous ring of 

 5 radial and 5 interradial pieces. Otolithic vesicles absent. Stone-canal 

 never opening to exterior. Retractor muscles well developed. Respira- 

 tory trees always, Cuvierian organs only occasional^ present. 



Cucumaria Blainville (Fig. 177), 10 tentacles, feet in rows on the radii ; 

 Thyone Semper, feet over whole body ; Orcula Troschel, Phyllophorus 

 Grube, with an inner row of smaller tentacles ; Pseudocucumis Ludwig ; 



FlQ. 186. Pelagothuria natatrix (from Lang, after Ludwig), seen 

 from the aboral pole. 



