4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



Thus it is evident that in a number of crinoid genera, as in many 

 groups among the fixed and arborescent marine organisms, we have 

 to deal not only w^ith the usual and well-recognized geographical 

 variation and the formation of geographical " subspecies," but also 

 with the formation of " varieties " which are strictly comparable to 

 the accepted " varieties " among the plants. Such difficult and intri- 

 cate genera as Cratcegus, Aster, etc., have their representatives among 

 the crinoids, though in the latter the number of included species is not 

 by any means so large. 



Many plants form marked local varieties correlated with the 

 physical and chemical character of their immediate environment — the 

 type of soil, amount of sunlight, average temperature, amount of 

 moisture, etc. — and several of these varieties may often be found 

 within a relatively limited area, as in certain species of Aster. Many 

 crinoids do exactly the same thing as a result of local variations in 

 the food supply, the amount and kind of illumination, and the 

 temperature. 



FORM AND STRUCTURE OF THE CRINOIDS 

 Typically a crinoid is rather abruptly divided into a slender and 

 more or less flexible column which supports at its summit a pen- 

 taradiate head or crown (figs. 39-42) the five divisions of which 

 each may bear anywhere from one (figs. 42, 50) to nearly forty (fig. 

 17, left), but most commonly two (figs. 39, 43, 46-48), flexible arms 

 along the ventral side of which, giving off branches to the slender 

 alternating lateral appendages or pinnules, runs a narrow ciliated 

 groove (fig. 36, left), the grooves from all the arms of each division 

 uniting and running in a single groove across the so-called ventral 

 disc to the mouth (figs. 4, 5). The cilia in these grooves pick up and 

 convey to the mouth the minute plankton organisms, both animal and 

 vegetable, which serve the crinoid as food. 



A single living crinoid (Holopiis), lacking the column altogether, 

 is attached directly by the crown, the basal portion of which becomes 

 much elongated. In the Plicatocrinidse the column continues through- 

 out life to add new segments just beneath the crown and never, so to 

 speak, matures. In the large group to which all the living crinoids 

 except the Plicatocrinidse belong the column increases to a definite 

 size, the topmost columnal then enlarging and becoming permanently 

 attached to the crown, forming a sort of apical plate. This 'is the 

 general law which, like all of nature's laws, must be liberally inter- 

 preted, for it is subject to curious modifications. In some types, as in 

 Bathycrinus, maturity comes slowly and several of these topmost 



