TYPE ECHINODERMA. 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, Diadema, 

 A rbacia , Strotigylocentrotus. 



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



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

 masticatory apparatus present. Clypeaster, EcJiinarachnius, 

 Mellita. 



3. Order Petalosticha. 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 Elasipoda. Bilateral symmetry well marked ; stone-canal 



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

 pidia. 



2. 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, Psohts, Thy- 

 one. 



Tentacles pinnate. Rhopalodina. 



Tentacles peltate. Holothuria, Miilleria. 



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



not opening to exterior ; tube-feet wanting. 

 Kespiratory trees present. Molpadia 

 Respiratory trees wanting. Synapta, Chirodota. 



LITERATURE. 



GENERAL. 



Johannes Muller. Ueber die Larven und die MetaphorpJiosen der Echinodermen. 



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

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



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

 W. P. Sladen. Tlie Homologies of the Primary Larval Plates in the Test of the 



Brachiate Eehinoderms. Quarterly Journal Microscop. Science, xxiv, 



1884. 

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



Echinodermen. Zeitschr. fill 1 wisseusch. Zoologie, XLVI, 1887. 

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



