MONOGRAPH OF THE EXISTING CRINOIDS. 481 



and show between them remnants of the earlier lumen. Already some of the inner 

 cells appear about to disintegrate. 



Seeliger found that the sacculi on the pinnules in course of formation on a 

 regenerating arm tip develop exactly as the first sacculi of the larvae. 



Connective tissue. The remaining mesenchyme cells of the larva, which take 

 part neither in the construction of the calcareous plates nor in the formation of 

 the sacculi, are collectively considered as composing the connective tissue. 



No sharp line can be drawn between these cells and those which lay down 

 the calcareous framework, as it is impossible to determine which of the cells 

 and cell groups within the interstices of the plates serve exclusively as connective 

 tissue for the occupation of the spaces, and which serve only in secreting lime. 

 It is possible that, in general, the more rounded mesenchyme cells may possess 

 the latter function; but, on the other hand, it is also possible that the branched 

 or spindle-shaped cells between the calcareous trelliswork in the column may 

 exercise both functions. 



As already explained, the mesenchyme cells lying under the ectodermal epi- 

 thelium merge with it and form a more or less uniform tissue in which the two 

 original layers are no longer to be distinguished. Only from their position can 

 it be inferred that the cells situated more deeply originate from the mesenchyme, 

 and that the outer, which are often spindle shaped and placed at right angles to 

 the surface, belong to the ectoderm. 



As the plates become more and more extensive the deeper layers of mesen- 

 chyme cells gradually disappear to sink into the interstices of the calcareous 

 skeleton and apparently also to take part in the enlargement of its various 

 elements. 



Thus in the calyx of the older larvae the much-thickened calcareous plates 

 are in general covered only by a layer of ectoderm cells, mostly placed at right 

 angles to the surface, all of the subepithelial cells being inclosed within the cal- 

 careous framework. Only in the radial planes between the plates does the body 

 wall preserve its original appearance. In the stem only over the tapered ends 

 of the masses of longitudinal bars arising in each direction from the primitive 

 rings and between the individual columnals is the original character of the body 

 wall retained. 



In the mesenchyme of the column connective tissue fibers are also found, 

 which in part may be traced back to the structures already noticed in the free- 

 swimming larvae, and in part are of later origin. 



The fate of the fibers is the free-swimming larvae interpreted as muscular 

 has not been determined; it is possible that they have broken down. No muscle 

 fibers have ever been demonstrated in the young attached larva, though possibly 

 they exist but have not been differentiated from other connective tissue elements. 

 If any occur they could only be individual fibers, for the later musculature arises 

 quite independently. 



