The Actiiiiae of the Plate CoUection. 297 



diaraeter of 4.5 — 5.0 mm. In one colony, seatecl upon a stone, all 

 the indiviciuals Avere considerably sm aller, biit all gradations of size 

 occurred and the flgures given above seem to represent the di- 

 mensions of the average mature polyps. 



The colonies are of a sandy grey color, due to the incrusting- 

 raaterial, and as this lessens in araoimt at the snmmit of the colnmn, 

 this has a somewhat paler shade. An accompanying label states 

 that the tentacles in life were rust-red. 



Structiire. The coliimn wall and coenenchyme were heavily 

 incrusted with particles of sand, rendering- the preparation of satis- 

 factory sections a matter of some difficulty. Almost the entire thick- 

 ness of the mesogloea is involved by the incrnstation throughout 

 the greater portion of the colnmn, but toward the summit the 

 particles are less numerous and more limited to the outer layers 

 of the wall. A distinct cuticle was present and the ectoderm was 

 much thiuner than the mesogloea. The latter contained rather 

 numerous cell Islands, but there were no indications of canals and 

 a ring sinus was wanting. The sphincter was imbedded in the 

 mesogloea and in its uppermost third consists of an open network, 

 while proximally it is composed of a Single row of more or less 

 spherical cavities, which gradually approach the endodermal surface 

 of the mesogloea (Fig. 86). 



The tentacles and disc were quite free from incrusting particles 

 and the ectoderm of the tentacles was very thick compared with 

 the mesogloea. The stomatodaeum possessed a deep siphonoglyph, 

 whose ectoderm was composed of closely packed slender cells, there 

 being none of the gland cells which were present on the general 

 surface of the stomatodaeum. The mesogloea of the siphonoglyph 

 is much thicker than that of the stomatodaeum proper and the latter 

 has a smooth ectodermal surface, not being raised into longitudinal 

 ridges. 



The mesenteries are arranged on the macrotype and are about 

 thirty-two in number, i. e. they are arranged in sixteen pairs. Their 

 mesogloea is thin and the longitudinal musculature but feebly deve- 

 loped. The mesenterial filaments, on the other band, both in their 

 ciliated and glandulär portions are very extensively developed. The 

 perfect mesenteries bore abundant sexual cells, which in the two 

 individuals examined, taken from the same colony, were sper- 

 matozoa. 



20* 



