768 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Summary. 



Pseudoplexaura crassa, an alcyonarian of the group Gorgonacea, 

 shows the character of Gorgonacea so far as regards the regions 

 recognizable in cross sections of the branches; the branches have a 

 central horny axis, a thick coenenchyma and an outer zone of polyps. 

 The horny axis shows a marrow composed of large chambers arranged 

 end to end, and a peripheral layer of smaller less regularly shaped 

 ones arranged side by side and irregularly overlapping one another. 

 The coenenchyma has, not far from the axis, a region of large longi- 

 tudinal canals. These are sometimes prolonged at their tips into 

 solenia. The polyps are long, and have ten to twelve pairs of pinnae 

 on each of their tentacles. They are crowded, so that when expanded 

 they hide the coenenchyma. Groups of small, crowded, irregularly 

 stellate, purple spicules occupy the deeper parts of the coenenchyma, 

 and larger, spiny and spindle-shaped, usually white spicules are in its 

 outer part. No spicules are found in the polyps. 



The ectoderm has the usual cover cells, nematocysts, sense cells, 

 and interstitial, ganglion, and muscle cells. Small nematocysts are 

 found in the ectoderm of the polyp's column, tentacles and stomo- 

 daeum. Large ones in considerable numbers are grouped into bat- 

 teries in the coenosarc. Ganglion cells are very few, and muscle cells 

 are found on the oral side of the tentacles and disk only. In the 

 ectoderm of the coenosarc between the polyps some of the ectoderm 

 cells have each a prominent supporting fiber, which runs from near 

 the nucleus perpendicularly to the mesogloea. 



The mesogloea is thin, except in the coenosarc regions, where it is 

 very thick. Cords of cells like the interstitial cells of the ectoderm 

 can be traced from the ectoderm to the deeper layers of the mesogloea. 

 In these cords there are partly formed and fully formed nematocysts, 

 spicule cells, and cells having an irregular shape and either containing 

 granules or destitute of them. These irregularly shaped cells form a 

 transition to the jelly-secreting cells, which are small and have many 

 long branches. Large spicules are produced by characteristic secret- 

 ing cells with large granules and one to many nuclei. Spheroidal 

 nutrition cells occur in many colonies, but these are found in both 

 ectoderm and endoderm; they probably originate in the endoderm 

 of the canals which form a network through the mesogloea. 



The endoderm cells are of characteristic form, being united with 

 each other at the proximal and distal ends, but, in fixed material. 



