Class 2. Anthozoa. Order 1. Alcyonaria. .115 



may be seen small pits, in wMoh the free, soft portions of the individuals were 

 situated. In some forms (Bhipidigorgia) the branches of the colony all lie in 

 the same plane and pai-tiaUy coalesce, so as to constitute a perforated plate. 

 Chiefly in warmer seas, some species in the Mediteiranean. Isis approaches 

 the Gorgonidse, but the axis consists of a series of calcai-eous joints limited 

 by homy transverse septa. One species in the Mediterranean. 



4. The Precious Coral {Corallium ruhrum). Branching colonies, with 

 hai'd, calcai-eous axis. Both axis and coi-tex are of a beautiful red coloiu', the 

 free portions of the individuals are white. In the Mediten-anean. 



5. Sea-feathers (genus Pennatula and others). The colonies consist of a 

 lower naked stalk and an upper, broader, often feather-Uke region, from which 

 the individuals, project. Within the axis of the colony there is a calcareous 

 imbranched rod. They ai-e fotmd at the bottom of the sea, with the stalk 

 sticking into the ground, and if disturbed, they bore their way deep down. 

 The phosphorescent, red, feather-like Pennatula phosphorea occurs in the 

 Noi-th Sea. 



Order 2. Zoantharia {Foly actinia). 



The number of mesenteries is either a multiple of six, or a 

 large indefinite number. The tentacles, which are simple, almost 

 always correspond in number with the mesenteries, and in most forms, 

 as in the Alcyonarians, there is a skeleton which is, however, very 

 different in character. It is limited exclusively to the lower region 

 of the body, and consists of a continuous, very jjorous or thick 

 mass of calcium carbonate, whilst the upper portion is entirely 

 without skeletal structures. The skeleton corresponds closely with 

 the general form of the animal, and usually consists of the 

 following parts: a discoid basal- plate, bearing a tubular 

 theca and a number — 12, 24, 48 — of radial septa which are 

 fused with the theca and basal-plate, and often ■ndth each othei' 

 also. 



From this description it might be supposed that the theca represented a, 

 calcification occuiring in the outer body- wall ; the septa, similar hardenings in 

 the mesenteries and the basal plate in the lower discoid portion of the animal. 

 As a matter of fact, however, this is not the case. In the fii-st place, it mxist be 

 noticed that the skeleton is not developed in the mesoglsea as was formerly 

 believed, but is an external secretion of the ectoderm. As soon as the 

 tiny free-swimming larva settles, it secretes, upon its attached sm-face, a 

 thin calcai-eous disc, which becomes, later, the basal-plate. Next, the 

 septa appear upon tliis plate, gradually growing taller and more plate-like, 

 untU, covered by the soft tissues of the pedal disc, they project into the 

 gastric cavity, between the twelve soft mesenteries which have already 

 been formed. A further secretion from the attached sui-face now appears, 

 as a ring-like ridge upon the basal plate, the beginning of the theca; the ridge 

 grows into a tall cylinder, covered like the septa by a fold of the basal disc, and 

 also projecting into the coelenteron ; it lies some little distance within the soft 

 body- wall. Other septa may develop later between those of the original set, and 

 these again have nothing to do with the mesenteries. It may also be remarked 

 that, in some Corals, the basal-plate may project up on to the lateral wall of the 



