J. E. Dueeden — Jamaican Aetiniaria : Part I. — Zoantheae. 337 



Column-wall (PI. xviii.a, fig. 1). — A cuticle and sub-cuticla are present, the 

 former having much adhering matter, such as diatoms ; the sub-cuticla is in 

 places minutely convoluted, evidently as a result of the excessive shrinkage of 

 the mesogloea. This is more especially seen in longitudinal sections. 



The ectoderm of the column-wall is broad above, but very narrow below. 

 It is much vacuolated, with only a small amount of cellular tissue remaining in 

 the form of strands passing from the outer to the inner boundary ; rarely a 

 connecting strand of mesogloea is seen. Some examples are not so highly 

 vacuolated, particularly in the lower part. Abundant medium-sized, oval, non- 

 staining nematocysts are present, the inner thread showing distinctly ; pigment 

 granules are met with at its internal boundary. 



An irregular layer of spherical lacunae appears in the mesogloea, immediately 

 below the ectoderm. Proximally the empty spaces extend further into the meso- 

 gloea. The mesogloea is broad in the region of the sphincter muscle, but narrows 

 much below. Cells with long, fine processes are distributed sparingly throughout ; 

 delicate fibrils can also be easily seen passing from the ectoderm to an irregular, 

 much broken, encircling sinus. The latter, situated either about the middle or 

 very near the endodermal border of the mesogloea, is formed of spaces varying 

 in dimensions and form. They contain a small quantity of cellular tissue, and 

 are connected with one another by larger or smaller canals, and also by canals 

 with the ectoderm and the endoderm. The cells are multipolar in character. 



The endoderm is low and contains abundant zooxanthellae ; nematocysts, 

 similar to those in the ectoderm are present, and a weak circular muscle. 



Sphincter muscle (PL xvur. A, fig. 1). — The sphincter muscle is mesoglceal 

 and double, the two halves being distinctly separated. The upper or distal is 

 smaller, and located in large irregular cavities extending almost across the 

 mesogloea, diminishing both proximally and distally ; the lining of muscle cells 

 is thin. The lower or proximal part of the muscle is contained in a large 

 number of small, scattered, mostly circular, mesoglceal cavities ; the distal ones, 

 however, are elongated, more like those in the upper. The muscle fibres are 

 very small in section, and only a little loose tissue is present in addition. 



Tentacles. — The ectoderm of the tentacles is without cuticle or sub-cuticla, 

 and shows two kinds of nematocysts — an outer thick zone of the usual narrow 

 form, and an occasional medium-sized, oval-shaped form similar to those in the 

 ectoderm of the column-wall. The mesogloea is very thin. 



The endoderm is well developed, and crowded with zooxanthellse. Both 

 the ectodermal and endodermal musculatures are weak. 



Disc. — The ectoderm of the disc is almost devoid of nematocysts. The 

 mesogloea is a little thicker, and the endodorm much thinner than in the tentacles. 

 The endodermal muscle is seen in longitudinal sections. 



TBANS. EOT. DTJB. SOC, U.S. VOL. VI.. PART XTV. 3 F 



