no ZOOLOGY FOR MEDICAL STUDENTS chap. 



with stiff translucent mesogloea which gives sohdity to the colony as a 

 whole. This mesogloea forms a kind of packing between the various 

 coelentera (Fig. 48) and it is further traversed by a coarse network 

 composed of tubes (t) and strands (si) of endoderm which link up the 

 various coelentera with one another. The mesogloea is further colonized 

 by numerous amoeboid or mesenchyme cells which have wandered into it 

 from the ectoderm. These immigrant ectoderm cells are functionally 

 scleroblasts, i.e. their function is to produce skeleton. They settle down 

 in the mesogloea and secrete spicules (Fig. 48, sp) of calcium carbonate 

 of a characteristic thorny appearance though they vary much in their 

 shape. These spicules are specially crowded together in the surface 

 layer of the colony converting this into a harsh protective rind and 

 giving the surface its characteristic colour. 



Alcyonaria 



The group Alcyonaria includes a great variety of marine creatures 

 agreeing with Alcyonium in their main features — the number (8) and 

 shape (pinnate) of the tentacles^ the eight mesenteries each with a muscular 

 thickening on its ventral face, the presence in the mesogloea of a 

 spicular skeleton formed by immigrant ectoderm cells, and as a rule the 

 formation of a colony by a process of budding. 



Before leaving the group it will be well to notice one or two interesting 

 departures from the condition seen in Alcyonium as regards the skeleton. 



The well-known " Red Coral " is formed by an Alcyonarian named 

 Corallium, the colonies of which differ conspicuously from those of 

 Alcyonium in their being slender and much branched, and in their bright 

 red colour due to the colour of the spicules. In Alcyonium the spicules 

 are specially crowded together towards the surface of the colony : in 

 Corallium a similar crowding together takes place along its axis, where 

 however the spicules become actually cemented together to form a solid 

 rod-like mass which is " Red Coral." 



Another Alcyonarian the colonies of which have slender branches 

 supported by an axial skeletal rod is Gorgonia, but in this case an 

 interesting difference exists in the mode of formation of the skeletal 

 rod. The young commencing colony secretes a plate of horny material 

 between its base and the substratum to which it is attached. As the 

 colony grows, more material is added by the secretory activity of the 

 basal ectoderm, so that the plate becomes converted into a little hillock, 

 still more is added till it forms a cylindrical pillar, and this being added 

 to continuously becomes gradually converted into a long rod over 



