360 An interesting Structural Analogy. 



general occurrence in the crinoids of the deeper waters than 

 in those of the shore-line, and, as they and more or less 

 similar structures are of quite general occurrence among 

 fossil forms, the crinoids of the deep sea have been considered 

 more primitive than those of shallow water. But the question 

 of the development or suppression of side- and covering- 

 plates is susceptible of a much more logical explanation. 

 The crinoids of the deeper levels must depend for their food 

 upon the rain of minute dead organisms from above; few, if 

 any, small organisms live among them ; the littoral crinoids, 

 on the other hand, are surrounded by multitudes of living 

 microscopic animals which they intercept by means of their 

 arms and pinnules, and can derive little or no benefit from 

 dead ones, as, aside from the fact that there are comparatively 

 few of them, they are undoubtedly promptly devoured by 



Fisr. 1. 





The scutes of the middle toe of a specimen of Bonasa umbellatuH, 

 shot in November. 



Fig'. 2. 



A portion of a pinnule from a crinoid belonging to the family Thalasso- 

 metridae, showing the side- and covering-plates bordering the pinnulars. 



other small predacious organisms, such as the minute Crustacea 

 or the bacteria. While a dead animal falling upon the upward 

 slanting extended covering-plates would roll clown them into 

 the ambulacral groove, where it would come within reach of 

 the cilia, a living animal on coming in contact with these 

 plates would instantly back away, as Paramecium does on 

 encounteiing the sides of a glass dish. Thus any increase in 

 pinnule area resulting from the formation of laterally exten- 

 sible plates, while of the vei-y greatest service to species 

 inhabiting water where only dead organisms are available, 

 would not be of the slightest use to species existing in the 



