BASAL PLATES IN CRINOIDEA CAMERATA 501 



some of the slighter morphological changes found in the Camerata 

 will be considered in the discussion of morphological principles. 



Having noted the more conspicuous changes through which 

 the basal cup has passed, a detailed study of the processes involved 

 in these may be undertaken. 



DETAILED STUDY OF PROCESSES ACTIVE IN PLATE EVOLUTION 



The processes active in the evolution of crinoid plates, especially 

 the basals and radials, may be divided into two broadly separated 

 though often co-operating groups : (1) those which do not necessarily 

 modify the relation of contact and position of the plates; and (2) 

 those which do modify these relations. The discussion of the first 

 group of processes includes: (a) symmetrical growth; (&) sym- 

 metrical reduction ; and (c) anchylosis. The second group includes : 

 (a) reduction and compensating growth; (b) enlargement and 

 compensating reduction; (c) plate division; (d) plate migration; 

 (e) plate interpolation; and (/) anchylosis. 



I. CHANGES NOT PRIMARILY MODIFYING THE RELATIONS OF PLATE 

 CONTACT AND POSITION 



a) Symmetrical growth. — Plate growth in the Echinoidea and 

 Crinoidea 1 is due to the deposition, by amebod cells, of crystalline 

 calcium carbonate, or calcium and magnesium carbonate 2 in re- 

 ticulate pattern in the mesenchyme. Three, or perhaps more, of 

 the ameboid calciferous cells fuse by means of pseudopodia into a 

 Plasmodium or reticulate tissue. 3 There the pseudopodia meet, 

 the protoplasm forms a small calcareous nodule (intracellular secre- 

 tion, according to Theel; extracellular, according to Semon), which 

 gradually increases along the pseudopodia, forming a triradiate 

 spicule. By further branching and anastomosis, the branches of 

 the spicule meet and fuse at the tips of their processes (Fig. 3, 

 No. 3), thus building up a hard tissue (stereom), showing a strongly 



1 Although no observations have been made upon the early details of plate deposi- 

 tion (stereom formation) in the Crinoidea, the growth of the plate from the primary 

 spicule on so closely parallels that in the Echinoidea that there can be no doubt con- 

 cerning the method of formation. 



2 Composition of crinoid skeletons: ref. 25, p. 31; ref. 14, p. 488; ref. 17. 



3 Stereom formation: ref. 34. 



