86 ZOOLOGY. 



Silurian Heliolites, form, according to Moseley, a new 

 family of Alcyonarians in which the corallum consists of an 

 abundant tubular coenenchym, with calicles having an 

 irregular number of pseudo-septa, which do not, how"ever, 

 correspond with the membranous mesenteries. The polyps 

 are completely retractile, with the tentacles when retracted 

 introverted. The mouths of the sacs lining the coenenchy- 

 mal tubes are closed with a layer of soft tissue, but com- 

 municate with one another and with the calicular cavities 

 by a system of transverse canals (Moseley). Heliopora cceru- 

 lea grows on coral reefs at the Philippine Islands and at 

 Singapore. 



In the family of sea-fans (Gorgonidm) the coral-stock is 

 horny or calcareous, branching tree-like, or forming a flat 

 network. The short calicles of the single retractile polyps 

 stand perpendicularly to the axis, communicating by longi- 

 tudinal vessels and branching canals. Grorgonia {Rhipigor- 

 gia) flabdlum Linn, is red or yellow and abundant on the 

 Florida reefs. In the Arctic seas and the deeper, colder 

 waters of the Newfoundland Banks and St. George's Banks, 

 Primnoa reseda (Pallas) and Paragorgia ariorea (Linn.) 

 grow ; the latter being of great size, the stem as thick 

 through as one's wrist, and the whole corallum over five feet 

 in height. 



In the family of sea-pens (Pennainlidce) the polyp-stock 

 is free, growing in the sand or mud, usually with a horny 

 axis supporting the polyps, and capable of moving at the 

 base. In Pennatula, or the sea-pen, there are secondary 

 branches in which the polyps are situated ; this polyp is 

 phosphorescent ; one species {P. aculeata Danielsson) lives 

 in deep water. An Arctic form, Umhellularia groe?iIandica, 

 is a gigantic form, growing about four feet high, in from 

 three hundred to two thousand fathoms. The species of 

 Reyiilla are kidney-shaped, with the polyps placed on one 

 side. Reiiilla retiiformis Cuvier is a rich purple species, 

 occurring in the sand at Charleston, S. 0. According to 

 Agassiz, this animal is remarkably phosphorescent, emitting 

 " a golden green light of a most wonderful softness." 



While coral reefs are in part composed of Alcyonarians, 



