THE APODOUS HOLOTHURIANS 127 



MYRIOTROCHTN^ Ostergren, 1898p. 



Tentacles digitate, much as in Chiridota, but -n^eaker, with 2-8 digits on 

 each side.' Cartilaginovis ring, light-detecting and gustatory organs, and true 

 ciliated funnels wanting. Polian vessel and stone-canal single. Calcareous de- 

 posits in the form of wheels, always with more than six spokes and never col- 

 lected in wheel-papillfP. Other calcareous deposits very rare. Sexes separate. 

 Size small. Distribution, arctic and subarctic, rarely north temperate. 



This well-characterized little group has been made known to science almost 

 exclusively through the efforts of the Scandinavian zoologists — Danielssen, 

 Koren, M. Sars, Theel, and Ostergren — and to the last two in particular are we 

 greatly indebted for careful and accurate work. All of the Myriotrochinse are 

 so small that they would be easily overlooked by careless workers, and as they 

 occur in mud and sand, often at considerable depths, it is not strange that, 

 with the exception of the largest and commonest species, very few specimens 

 have l^een found. The group has for many years been supposed to consist of 

 three monotypic genera, but the admirable work of Ostergren has shown ns that 

 one of these genera contains at least four species. None of the terms used in 

 the keys or descriptions require any explanation. 



Key to the Genera of JlYRioTROCHiN.Ti:. 



.-1. — Wheels of only one kind, witli 10-25 spokes. 



liim of the wheels witli numerous large teeth projecting liorizontally inward (plate 



VIII. figs. 17, 18, 19, 22) Mtriotrochus 



Eim of the wheel with scattered, somewhat pointed knohs, not projecting horizontally 



inward (plate viii, fig- 13) Trochodehjia 



.4.4. — Wheels of two kinds, with 8-11 spokes (plate viii. ligs. 4 and 5) Acakthotrochus 



MYRIOTROCHUS Steenstrup, 1851. 

 Oligofrochus M. Sars, 1866. 



Tentacles 10 or 12. Calcareous deposits consist of wheels with 10-25 

 s])okes, and with numerous large, flat, sharp teeth in-ojecting horizontally in- 

 ward from the rim; these wheels vary greatly in size and in abundance, but 

 all have the same general appearance. No other calcareous deposits in the 

 skin and rarely in the tentacles. 



Our knowledge of this genus has been revolutionized by Ostergren 's re- 

 cent jiapers ( :0o, -.Ooa, and :05&). Although two of the species have 10 tenta- 

 cles and two have 12, it does not seem to be possible to divide the genus into 

 two natural groujis, either by that or any other character. 



Key to the Species of .AIyuiotrochus. 



A. — Tentacles 12. Size moderate, 30 mm. or more. 



Wheels numerous all over the dorsal surface, infrequent or wanting venirally, 140- 

 330 /x in diameter, with 12-25 (average about 17) spokes and 16-35 (average about 

 26) teeth (plate vni, fig. 22) rinkii 



'Figures 16 and 21, plate viii, give very erroneoas representations of the tentacles of Myriotrochus. 



