648 ECHINODERMATA 



in 1776 described a considerable number of species, all of which he in- 

 cluded in the genus Holothuria, as did other authors of his time. Cuvier 

 in 1817 also placed all the known species in this genus, which he first 

 put in the class Echinodermata, the relationship of holothurians to star- 

 fish and sea-urchins not having been generally recognized before his time. 

 Goldfuss (1820), following Oken, divided the group into several genera, 

 but Jager (1833) again united all in the single genus Holothuria. He, 

 however, subdivided it into three subgenera and these into tribes, and his 

 whole arrangement may be considered the foundation of the modem 

 classification. Of great importance in more recent times have been 

 Semper's work on the holothurians of the Philippines (1867) and the 

 anatomical and systematic studies of Selenka and Ludwig. 



The class contains two orders and six or eight families, with over 

 500 species. 



Key to the orders of Holothurioidea: 

 Oi Ambulacral feet present, or if wanting, respiratory tree present. 



1. ACTINOPODA 



Ca Ambulacral feet and respiratory tree both absent 2. Paractinopoda 



Order 1. ACTINOPODA. 



This order includes the majority of sea cucumbers and all those in 

 which the oral tentacles spring from the radial canals alone; ambulacral 

 feet are present except in the Molpadiidae: 7 families. 



Key to the families of Actinopoda here described : 



Oj Ambulacral feet present. 



&i Oral tentacles branched only at tip 1. Holothuriidae 



62 Oral tentacles much branched 2. Cucumeriidae 



Oa Ambulacral feet absent 3. Molpadiidae 



Family 1. HOLOTHUEIIDAE. 



Holothurians usually elongated and more or less cylindrical in shape, 

 which have 18 to 30 (usually 20) oral tentacles, the branches of each of 

 which form a frilled and expanded end; tentacular ampullae present; 

 genital pore outside the ring of tentacles ; respiratory tree well developed : 

 6 to 8 genera and about 200 species, which are found principally in the 

 tropics, especially in the Indian and Pacific Oceans; about 13 species are 

 used by the Chinese for food, and enter into the commerce of the East 

 under the name of trepang. 



Holothuria L. Twenty oral tentacles (occasionally more) ; am- 

 bulacral appendages scattered over the entire body; calcareous bodies 

 in body wall numerous and very diverse in form: over 100 species, at 

 least 4 of which occur on the Florida coast. 



