128 CLARK: BATHYMETRICAL DISTRIBUTION OF CRINOIDS 



It should, perhaps, be emphasized that, strictly speaking, the 

 bathymetrical distribution of any animal type is of itself without 

 biological significance. The only factor correlated directly with 

 increase in depth, other than the decrease in illumination in the 

 upper strata, is the increase in pressure, and increase of pressure 

 has never been shown to exert any appreciable influence on the 

 distribution of the higher invertebrate types either directly, or 

 indirectly through the inhibition of the physiological processes. 



The determining factor in the bathymetrical distribution of 

 marine animals is the decrease of temperature with depth, and 

 the study of the bathymetric distribution of any large group not 

 directly dependent upon plants for food is in reality the indirect 

 study of its thermal distribution. As our temperature observa- 

 tions in any one group are usually comparatively few, while our 

 bathymetrical records are numerous, we are able to discuss to 

 advantage the bathymetrical distribution of the component 

 types in any unit, while at the same time we are unable to con- 

 sider similarly the thermal distribution of the same types. But 

 we must always remember that in discussing the bathymet- 

 rical distribution of a subfamily or higher group we are really 

 considering its thermal distribution, and our bathymetric 

 records may be readily transposed into thermal records by means 

 of comparisons with tables showing the decrease in the tempera- 

 ture in the sea according to latitude and depth. 



The average range in depth of the families of recent crinoids 

 (excluding the Stephanometridae, Tropiometridse and Holopo- 

 didse, monotypic, and the Apiocrinidae and Phrynocrinidse, in- 

 sufficiently known), calculated as the average of the ranges of 

 all of the included genera, represents a very varying percentage 

 of their total range — from 23 per cent in the Capillasterinse to 

 71 per cent in the Pentametrocrinidse. 



The sequence of the families according to the relation of the 

 average to the total range in depth is as follows: 



