THE APODOUS HOLOTHURIANS 187 



Molp^adia 

 Ceraplectana 



Caudina 

 \ / 



Aphelodactyla 



// 



Acaudina 



Gephyrothuria 



^ 



Himasthlephor a 



15-tentacled Cucuraarian 



Ancestor 

 GEOGRAPHICAL DISTRIBUTION. 



In considering the distribution of the foregoing genera we will use the 

 same system that served us in the discussion of the Synaptidae, even though a 

 large proportion of the Molpadiidas are abyssal forms. We are at once struck 

 by the fact that there is no one particular region characterized by an exceptional 

 number of genera, although three occur in the Pacific Boreal subregion and 

 four in the Indo-Pacific region; of the latter, two are characteristic, but of the 

 former there are none, although we might ignore the depth at which Cera- 

 plectana occurs and consider it a fourth and characteristic genus. Besides 

 Ceraplectana, two genera are exclusively Abyssal and two others occur in that 

 region. The most widely distributed genus, Molpadia, occurs in practically all 

 parts of the world where collections at depths of 200 m. or more have been 

 made, and Caudina also has a wide range, though it is properly a littoral genus 

 with only two abyssal species, and a distribution much like Chiridota (see p. 

 133) ; but Molpadia is strictly littoral only in the case of a very few species, and 

 chiefly in the Arctic or Antarctic regions. The following table will show these 

 facts of distribution in a convenient way ; an * indicates that the genus is char- 

 acteristic of the region, ** that it is not founfl elsewhere; the figures in paren- 

 theses indicate the number of species occurring in that region or subregion: 



Abyssal Region: Atlantic Boreal Subregion: 



* Molpadia (16). Eupyrgus (1). 



** Ceraplectana (1). Molpadia (4). 



** Gephyrothuria (1). Caudina (1). 



** Himasthlephora (1). Arctic Circumpolar Subregion: 



Caudina (2). Eupyrgus (1). 



Indo-Pacific Littoral Region: Molpadia (2). 



** Aphelodactyla (5). Pacific Boreal Subregion: 

 ** Acaudina (1). Eupyrgus (2). 



Caudina (1). Caudina (1). 



Molpadia (12). Molpadia (3). 



