232 THE STUDY OF ANIMAL LIFE CHAP. 



It is instructive to contrast the various kinds of corals. 

 Dead-men's-fingers with numerous jagged spicules of 

 lime in its flesh is just beginning to be coralline. Similar 

 spicules have been fused together in external tubes in 

 the organ-pipe coral. In the red coral of commerce 

 the calcareous material forms an axis around which the 

 individuals are clustered. Very different are the reef- 

 building corals, where the cup in which each individual 

 lived is more or less well marked according as it has 

 remained distinct or fused with its neighbours, and where 

 an image of the fleshy partitions of the sea-anemone-like 

 animal is seen in the radiating septa of lime. 



We begin the series of many-celled animals with 

 Sponges and Ccelenterates, partly because they are on 

 the whole simplest, but more precisely because their 

 types of structure are least removed from that two- 

 layered sac-like embryo or gastrula which recurs in the 

 life-history of most animals, and which we have much 

 warrant for regarding as a hint of what the first successful 

 many-celled animals were like. The Sponges and Ccelen- 

 terates differ from the higher animals : (1) In retaining 

 the symmetry of this gastrula, in being, like it, radially 

 symmetrical, and in so growing that the axis extending 

 from the mouth to the opposite pole corresponds to the 

 long axis of the embryo ; (2) in being two-layered animals, 

 for between the outer skin and the lining of the internal 

 food-cavity there is only a more or less indefinite jelly 

 instead of a definite stratum of cells ; (3) in having only 

 one internal cavity, instead of having, like most other 

 animals, a body-cavity within which a distinct food-cavity 

 lies. 



Regarding the layers of the body, we may notice here 

 that the outer layer of cells is called the ectoderm, the 

 inner layer the endoderm. In Ctenophores, which we have 

 noted as divergent, a third fundamental layer the 

 mesoderm takes the place of the rather indefinite 

 mesoglcea or middle jelly of other Ccelentera. Though 

 one would not perhaps suspect it, there are some striking 

 suggestions in Ctenophores of relationship with the 

 simplest flat-worms the Turbcllarians. 



