TYPE EOHINODERMA. 593 



twenty rows of plates ; ambulacra extending from oral to 

 aboral surfaces or else limited to aboral surface. 



1. Order Desmosticha. — Ambulacra all similar, extending from oral 



to aboral surfaces ; mouth and anus in centre of the oral and 

 aboral surfaces respectively ; masticatory apparatus present. 



With internal branchiae. Cidaris. 



With external branchiae. Asthenosoma, Salenia, Diadeina, 

 Arbacia, Strong ylooentrotus. 



2. Order Clypeastroidea. — Ambulacra all similar, limited to the 



aboral surface ; mouth in centre of oral surface, anus excentric ; 

 masticatory apparatus present. Clypeaster, Echinarachnius, 

 Mellita. 



3. Order Petalostieha. — Ambulacra more or less dissimilar, limited 



to the aboral surface ; mouth and anus both excentric ; masti- 

 catory apparatus wanting. Moira, Spatangus, Brissopsis. 

 V. Class HoLOTHUROiDEA. — Free forms ; body mostly elongate and ver- 

 miform ; mouth surrounded by circle of 10-30 tentacles ; skele- 

 ton usually of small scattered plates. 

 1. Order Masipoda. — Bilateral symmetry well marked ; stone-canal 

 frequently opens to exterior ; no respiratory trees. Deima, El- 

 pidia. 

 3. Order Pedata. — Bilateral symmetry as a rule not particularly 

 well marked ; respiratory trees present ; stone-canal not open- 

 ing to exterior ; tube-feet well developed. 

 Tentacles branched dendritically. Cucumaria, Psolus, Thy- 



one. 

 Tentacles pinnate. Rhopalodina. 

 Tentacles peltate. Holothuria, Mulleria. • 

 3. Order Apoda. — Bilateral symmetry not well marked ; stone-canal 

 not opening to exterior ; tube-feet wanting. 

 Respiratory trees present. Molpadia 

 Kespiratory trees wanting. Synapta, Chirodota. 



LITERATURE. 



GBNEKAL. 



Johannes Uliller. Ueber die Larven und die Metaphorphosen der EeMnodermen. 



Abhandl. KK. Acad. Wissensch. Berlin, 1848-55. 

 P. H. Carpenter. The Oral and Apical Systems of Echinoderms. Quarterly 



Journal Microscop. Science, xviii, 1878 ; xix, 1879. 

 W. P. Sladen. The Homologies of the Primary Larval Plates in tlie Test of t/ie 



Brachiate Echinoderms. Quarterly Journal Microscop. Science, xxiv, 



1884. 

 0. Hamann. Die wandernden Urkeimzellen und ihre Beifungsstaiien bei den 



EeMnodermen. Zeitschr. ftir wissensch. Zoologie, xlvi, 1887. 

 P. & F. Sarasin. Ueber die Anatomic der Echinothuriden und die Phylogenie 



