MONOGKAPH OF THE EXISTING CKINOIDS. 



499 



possible. All the cells are connected by protoplasmic processes and, as in younger 

 stages, lie in a homogeneous intercellular mass. 



In the middle and deeper layers of the body wall the cell forms are ex- 

 traordinarily diverse. They may be strongly rounded, star shaped, branched, 

 or spindle shaped, with pointed or gabled ends, each type being indiscriminately 

 distributed. Some, scattered and running in various directions, give the impres- 

 sion of muscle fibers, but whether they are in reality contractile elements has not 

 been definitely determined. The musculature of the hydroco?le suffices to explain 

 the movements in the larval calyx. 



In the column the mesenchyme shows approximately the same structure as 

 in the stage preceding. With the extraordinary elongation of the columnals the 

 importance of the connective tissue has diminished. 



Between each two columnals a layer of closely packed mesenchyme cells, 

 spindle shaped, with the strongly stained nuclei in the thickened central portion, 

 is found. Whether or not these cells are attached to the ends of the calcareous 

 segments has not been determined. 



The other mesenchyme cells of the column lie within the fascicles as a mass 

 of weak and pliant connective tissue, and are concerned with the formation of 

 the calcareous portion of the columnals. If any of the cells function purely as 

 connective tissue it is not evident from their structure, since all of the mesen- 

 chyme cells appear essentially the same. For the most part the cell bodies are 

 elongated in the direction of the major axis and are united through their branched 

 ends. The nuclei are sharply delimited. Within the calcareous substance cell 

 borders are very difficult to recognize, so that the nuclei appear to lie in a plasmatic 

 reticulum. 



The axial canal, passing through the central portion of all the columnals, is 

 now entirely occupied by the process from the axial organ, and the five processes 

 from the chambered organ surrounding it. The cord formed of these six struc- 

 tures is covered by mesenchyme cells only in the region of the articulations. 



Dorsal nervous system. At the junction between the calyx and the column, 

 about the chambered organ there is a considerable aggregation of mesenchyme 

 cells which does not take part in the formation of the calyx plates. They resemble 

 the usual connective tissue cells and are mostly rounded, with numerous star- 

 shaped, and some spindle-shaped. These cells eventually form the dorsal (meso- 

 dermal) nervous system the central organ of which surrounds the chambered organ. 



ANTEDON MEDITEKRAXEA. 

 Figs. 1182-1185, pi. 28, 1190, pi. 29, and 1194, pi. 30. 



Very detailed studies have been made on the embryology and development of 

 this species by Bury at Naples, and by Barrois at Villafranca, his material 

 coming from Villafranca and especially from Toulon. 



Both investigators found that all the embryos, not merely on the same mother 

 but on all the individuals from the same locality, brought in on the same day 

 are in the same developmental stage. 



