114 



Ciielentera. 



this becomes gradually more convex^ growing up witi. tlie colony; 

 it is secreted by tbe ectoderm and is a true cuticular structure 

 (c/. Fig. 68 G) . The axial skeleton of Isis wbicli is to a great extent 

 calcified, is similar in its relations. 



Pig. 68. Sections through young ooloniee of various Aloyonaria (diagrammatic). Tho 

 layers of the body indicated as in Pig. 67. A simplest mode of communication between 

 the individuals. JB young colony of Alcyonium. ditto of a Gorgonian axial skeleton, 

 figured black. — After v. Koch. 



It is of interest to notice the constant presence^ in various Alcyo- 

 narianSj of arrested individuals (siphonozooids) amongst the normal 

 persons (autozooids) . In the most extreme cases, they have no 

 tentacles and differ in other respects from autozooids; in other 

 cases the difference is less pronounced. Their chief function appears 

 to be the reception and ejection of water, and is thus respiratory. 

 They are very common among the Pennatulids, but are also 

 found in Alcyonium and Corallium ruhrum, although not in the 

 Gorgonidse. 



Of the forms belonging to this group, the following may be 

 mentioned : 



1. Dead Men's Fingers {Alcyonium digitatum) form yellow or wMtisli 

 leathery colonies of an irregular bulky shape, ■with short thick branches. The 

 gastric-cavities are continued back from the free, upper, soft portions of the 

 individuals, as slightly sinuous tubes reaching far down into the stem, and 

 are connected by fine canals. No axial skeleton. Common on English 

 ■coasts. 



2. Organ-pipe Coral (Tubipora) forms massive colonies, consisting of 

 long tubular zooids, arranged almost parallel to one another ; the individuals 

 of the colony are not connected by coenenchyme, but by transverse platforms, 

 containing a network of tubes communicating with the gastric cavities. In 

 each individual the spicules (with the exception of those in the upper soft portions 

 of the body) are united to form a hard tubular mass : in the transverse platforms 

 they are fused into calcareous plates connected with the tubes. In the Indian 

 and Pacific Oceans. 



3. The Gorgonidse (genus Gorgonia and others) form arborif orm colonies 

 with hard, dark, homy axes both in the main stem and branches ; the rest of 

 the colony covering the homy axis is termed the " cortex," contains numerous 

 spicules, and may be perforated by canals. On the sui-face of the dried colony 



