THE SEA-PENS. 



305 



THE ORDER ALCYONARIA. 



The numerous members of this order are well distinguished by having eight tentacles, 

 or tentacles in multiples of four, which are very regularly pinnate. They form a single row, or 

 cycle, and are enlarged at their base, and each communicates with one of the eight perivisceral 

 spaces around the stomach. Certain boat-shaped spicules are found in groups, at the base of the 

 tentacles and in the derm, and are often coloured. The mouth varies in its shape, but has not 

 a bilobate form. The stomach terminates, internally, in an orifice surrounded by a sphincter 

 (pylorus). Eight mesenteries project from its outer surface into the perivisceral cavity, and 

 reach the walls of the body to which they are fixed. Each inter-mesenteric space is continuous 

 with a hollow tentacle above, and below it communicates with the visceral chamber near the 

 pylorus. The visceral cavity beyond the pylorus is variable in its size ; in the genus Corallium 

 and in the Gorgonias it is short and rounded below, but in Alcyonium it is long, and narrow 

 in shape. The body is very soft and retractile at the upper part of this portion of the cavity, 

 but at the lower part the deraial tissues contain sclerites and spicules. A calcareous stem 

 often results, which may branch, and become thick and concentric in its structure, or spicules may 

 simply strengthen the integuments, and in the first family coral-like structure exists. 



The family HELIOPORIDJ*E contains the so-called Blue Coral (Heliopora ccerulea), which is found 

 on many Pacific coral reefs. The Heliopora has a massive, hard, calcareous skeletal structure, with 

 pores on its surface leading clown to tubules, which are crossed by tabulae or horizontal floors. The 

 soft parts cover the hard, and dip down within the pores and to the level of the uppermost horizontal 

 tabulae. The pores have little projections, like imperfect coral septa, and there may be from twelve to 

 sixteen. But at a slight depth in the calice Moseley, to whom we owe the anatomy and zoology of the 

 group, says the projections become eight in number, and in the living 

 animal a mesentery passes to each internal projection. The soft tissues 

 of this hard cellular mass are composed of an ectoderm, mesoderm, and 

 endoderm. The first is superficial, and is also prolonged to form a lining 

 to the stomach. The mesoderm has connective tissue, layers of cells, and 

 masses of fibrillar tissue, and the carbonate of lime of the skeleton is 

 produced in the first. The endoderm forms layers lining the centre of 

 the tubes of the hard parts, the calicles, and the interseptal spaces. There 

 are deep superficial canals on the top of the hard skeleton, communicating 

 with the calices of the pores, and they are lined with the three dermal 

 elements. The polype with its tentacles has not yet been seen expanded, 

 but Moseley has drawn the unexpanded condition, and has shown that 

 retractor muscles exist, which withdraw it into the pore, down to the 

 upper tabula. Yery small nematocysts occur in the ectoderm. There are 

 fight lobes in the unexpanded polype, and eight tentacles exist, and there 

 are evidences of short stout tubercles on them. Moseley found ova in 

 about three polypes out of a hundred, but no spermatozoa ; so in all 

 probability these tabulate Alcyonaria are unisexual.* The fossil genus 

 Heliolites, of the Palaeozoic age, is a close ally of the Heliopora, which 

 has itself been found fossil in Secondary rocks, f 



THE FAMILY PEXXATULIDJE.-TIIE SEA-PEXS. 

 These are free-swimming, more or less pen-shaped Alcyonaria, and 

 some live with their slender pointed root in the sand and mud, but they 

 are not fixed. Their surface is soft, and may have three kinds of polypes, 

 or zooids, upon it, continuous by their bases. They are connected with 

 the central stem, or axis, which is fistulose, and made up of horny and calcareous matter, traversed by 



* H. N. Moseley, F.R.S. : Report on Corals, Challenricr Expedition. 



t Allusion has been made to the Tabulata as a group already ; and Moseley's researches almost necessitate the placing of 

 the Favositidae of Milne-Edwards and Jules Haime amongst the Alcyonaria, so that the group is very old, and was Paleozoic. 

 Some forms, however, aro Bryozoa. 

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PENNATULA (PTEROEIDES) 

 SPIXOSA. 



