226 THE HOLOTHURIOIDEA 



first appendages of the water-vascular system. The radial canals 

 and podia (when present) are now formed, and the young Holo- 

 thurian assumes the adult form. Rarely the Auricularia stage is 

 omitted, the ciliated gastrula developing more or less directly into 

 the barrel-shaped larva. 



Very little is known of the extinct Holothurians. Some spicules 

 have been found, many of which belong to the Synaptidae, in 

 deposits ranging from the Carboniferous to Tertiary strata. 



With advancing knowledge the Classification of the Holothurians 

 has undergone many changes since it was first attempted at the 

 beginning of this century. It now appears to be firmly established 

 on deep-seated structural characters. In 1815 Oken divided the 

 few species then known according to the shape of the body, whilst 

 soon after Lamarck made use of the tentacles as a taxonomic 

 character, a system afterwards perfected by Grube, who founded 

 the family Aspidochirotae for forms with peltate tentacles (Holo- 

 thuriidae), and Dendrochirotae for forms with arborescent tentacles 

 (Cucumariidae). Cuvier and others followed Okcn, taking into 

 account the occurrence and distribution of the podia ; and Brandt 

 divided the Holothuria into Pedatae with podia, and Apodes 

 without podia. Selenka, in 1867, following Jaeger, formed the two 

 orders Pneumophora and Apneumona, the first for the modern 

 Holothuriidae, Cucumariidae, and Molpadiidae, provided with re- 

 spiratory trees or " lungs," the second for the Synaptidae without 

 " lungs." The"el adopted the orders Apoda and Pedata, adding the 

 order Elasipoda for the newly discovered Elpidiidae. Ludwig has 

 clearty shown that the classifications founded on the mere presence 

 or absence of podia or of respiratory trees are artificial; first, 

 separating off the Synaptidae, which differ in important respects 

 from all the other families, as the Paractinopoda, he divides the 

 remainder of the Holothurians, the Actinopoda, into five families, 

 as shown in the following table : 



ORDER 1. Actinopoda. Radial canals sup- 

 plying tentacles and podia. 



A. With respiratory trees. 



r \ w-fi, A- /FAM. 1. HOLOTHURIIDAE. 



(a) With podia. | FAM 4 CUCUMARIIDAE . 



(6) Without podia FAM. 5. MOLPADIIDAE. 



B. Without respiratory trees, 



(a) With podia. FAM. 2. ELPIDIIDAE. 



(6) Without podia. FAM. 3. PELAGOTHURIIDAE. 



ORDER 2. Paractinopoda. Neither radial 

 canals nor podia. Tentacles 

 supplied from circular canal. 



FAM. SYNAPTIDAE. 



