328 ECHINODERMATA [CH. 



So far as we have yet seen the Echinoderms seem to differ from 

 Coelenterates, which are also radiate animals, in the details of the 

 arrangement of the organs rather than in any fundamental features. 

 The skeleton no doubt is peculiar in being embedded in the skin : 

 but the spicules of the Alcyonaria occupy a similar position, although 

 they rarely cohere to form the definite rods and plates like those 

 characteristic of Echinoderms. When the soft parts are dissolved 

 away from one of these rods or plates by caustic potash it is seen to 

 consist of a delicate network of calcium carbonate ; and it is found 

 by observation of the developing young that such a plate is formed 

 by a little heap of cells coming together and secreting a cal- 

 careous rod between them : this rod then branches at both ends and 

 the branches bifurcate again so that the twigs of the second or 

 third degree approach each other and joining form a mesh, and this 

 process of bifurcating and joining is repeated until the plate or 

 spine is built up. The growth of the primitive rod into the mesh- 

 work is rendered possible only by the growth of the cells which shed 

 out the calcium carbonate. These cells remain throughout life, 

 more or less modified, as a kind of living network interpenetrated by 

 the skeletal one. 



When however we cut a star-fish open we see that the animal 

 Coeiom apparently consists of two sacs placed one within the 

 other. The innermost sac or alimentary canal opens 

 in the centre of the upper surface by a minute opening, the anus, 

 through which undigested matter is thrown out, and on the under 

 surface by the mouth. The space or sac which apparently surrounds 

 the digestive cavity of the star-fish is a true coelom: like the 

 coelom in a segment of an Annelid it has been formed by the union 

 of two sacs which in the embryo lay right and left of the digestive 

 tube. From its walls the muscles are developed, the generative 

 cells, and also the cells which give rise to the skeleton. Between 

 the outer wall of the body-cavity and the true external skin which , 

 corresponds to the ectoderm, there is a mass of more or less 

 gelatinous substance exactly corresponding to the jelly of a Medusa 

 or the connective tissue of an Arthropod, which constitutes the 

 substance of the body- wall. Into this material wander cells budded 

 from the wall of the coelom : these cells from their power -of move- 

 ment and appearance can be recognised as amoebocytes. It is 

 from these cells that the skeleton is formed in the way we have 

 described above : some of them, however, retain their primitive 

 character and wander about, probably carrying food to the various 



