Ill] ALCYONIUM 71 



ectoderm, which is merely tucked in at the mouth. Such a tube is 

 known as a stomodaeum 1 . The mesenteries, although they end 

 freely below, are attached to the sides of the stomodaeum above, 

 so that in this region the coelenteron is divided into a number 

 of compartments, each of which is prolonged into one of the hollow 

 tentacles (Fig. 31). 



A microscopic section of such a polyp shows us several other 

 interesting points. We see that we have to deal with the same 

 layers which we met with in Hydra, skin (or ectoderm) and 

 coelenteron lining (or endoderm). Between them, however, there is 

 the jelly, which was present as an exceedingly fine membrane in 

 Hydra, and which, greatly thickened, formed the substance of the 

 bell of the Medusa. This jelly is fairly thick in the minute sea- 

 anemone we are examining, and here contains cells which have 

 wandered into it from the ectoderm. Some of these cells have the 

 power of secreting thorny rods of lime, termed spicules. These 

 spicules are very abundant where the polyp merges into the general 

 surface of the colony, so that they form a kind of stiff protecting 

 crust round the base of the polyp and over the surface of the 

 colony from which the polyps rise. In the organ-pipe coral, 

 Tubipora, the spicules in the lower parts of the polyps are so 

 felted together that they form a set of parallel tubes, suggesting 

 the pipes of an organ ; only the upper part of the polyp, where the 

 spicules are not yet closely aggregated, being capable of movement. 

 We have spoken above of the colony as distinct from the polyps, 

 and this use of the word demands some justification. When we 

 were dealing with the Hydromedusae, we used the word colony in 

 the sense of the whole mass of the polyps which cohered together, 

 and which had arisen by the growth of one original polyp. Now 

 in Alcyonium and its allies, budding does not take place in quite 

 the simple manner in which it occurs in Hydra and its allies. 

 Instead of one polyp growing directly out of another, the coelen- 

 teron of the parent sends out a tube lined only by endoderm. This 

 tube grows, pushing the ectoderm before it; but, as between the 

 ectoderm and endoderm there is a thick jelly interposed, the endo- 

 dermal tube can branch without the ectoderm becoming indented. 

 Where the free ends of these tubes reach the surface, there fresh 



1 "I have proposed to designate this ingrowth... the stomodaeum (<TTOfj.6daiov, 

 like TrvX6Saiov, the road connected with a gateway) and similarly to call another 

 ingrowth which accompanies the formation of the second orifice (the anus) of 

 the enteron, the proctodaeum (irpuKT6s, the anus)." Kay Lankester. 



