IIO ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



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



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



g. 48) and it is further traversed by a coarse network 



: of tubes (/) and strands (st) of endoderm which link up the 



.rlentera with one another. The mesogloea is further colonized 



liv numerous amoeboid or mesenchyme cells which have wandered into it 



derm. 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 



Characteristic thorny appearance though they vary much in their 



,e. These spicules are specially crowded together in the surface 



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



I the surface its characteristic colour. 



ALCYONARIA 



The group Alcyonaria includes a great variety of marine creatures 



ing with Alcyoirium 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 



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



formation ot a colony by a process of budding. 



r.etore 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 



illiuiii, the colonies of which differ conspicuously from those of 

 . \l t -ytniiiun in their being slender and much branched, and in their bright 

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

 .ire sj)tcially crowded together towards the surface of the colony: in 

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

 lm\\ \< i the spicules become actually cemented together to form a solid 

 r>d -likr mass which is " Red Coral." 



Another Alcyuiiurian the colonies of which have slender branches 

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

 interest in- diticrcncr exists in the mode of formation of the skeletal 

 r"d. Tin- young commencing colony secretes a plate of horny material 

 hetuern its l.asr and the substratum to which it is attached. As the 

 more material is added by the secretory activity of the 

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

 >till mure is added till it forms a cylindrical pillar, and this being added 

 to continuously becomes gradually converted into a long rod over 



