METAZOA 129 



ment, either through the unregulated exchange of substances or by 

 unregulated stimuli. We shall see also, how the machinery which is 

 fashioned in this way varies in correspondence with the environment. 

 In the phylum to which Hydra belongs, the Coelenterata, the body 

 is always of the type just described, whatever form the sac or its layers 

 may assume, though the jelly may contain cells, sometimes plentiful, 

 of various kinds — muscle fibres, skeleton forming cells, and amoeboid 

 corpuscles — which have migrated into it from the ectoderm or endo- 

 derm. In all other metazoan phyla there is between ectoderm and 

 endoderm a third layer, the mesoderm, which usually is more bulky 

 than either of the other layers and forms the greater part of the body. 

 The phyla which possess this layer are known as Triploblastica — 

 three-layered animals — while the Coelenterata are Diploblastica. It 

 is true that the mesoderm is partly foreshadowed by the cells which 

 are present in the jelly of many coelenterates, but mesoderm is more 

 plentiful than the cells in the jelly generally are, it contains important 

 organs and usually definite systems of spaces (see p. 131), and its 

 rudiment appears very early in the development of the individual. 



Every triploblastic animal, however, passes through a stage — the 

 gastrula — in which it consists only of ectoderm and endoderm. Save 

 in this essential feature, the gastrulae of diff'erent animals may be 

 extraordinarily unlike, and, especially when the animal is developed 

 from a very yolky tgg, they are sometimes very difficult to recognize 

 as such; but where the gastrula is well formed, as in the familiar de- 

 velopment of Amphioxus or in that of a starfish (Fig. 438), its two- 

 layered wall may always be found to contain a cavity, the archenteron, 

 which possesses a single opening, the blastopore. The ectoderm and 

 endoderm are separated by a space, which is often a mere crack, but 

 may be much wider, and contains a fluid or a slight jelly. This space 

 is known as the blastocoele, and when, as in the cases cited above, the 

 gastrula arises by the dimpling-in (invagination) of the wall of a one- 

 layered hollow vesicle or blastula, the blastocoele begins as the cavity 

 of the blastula. 



The mesoderm, whose appearance converts the gastrula into a 

 triploblastic body, is not a single entity, but contains components 

 which originate in two diff'erent ways, namely: 



{a) Cells which migrate from ectoderm or endoderm, or from 

 mesoderm of the other kind, into the blastocoele; this kind of 

 mesoderm (Fig. 438, mch.) is known as mesenchyme, and is comparable 

 to the cells which invade the jelly of coelenterates. 



{b) Cells which constitute the wall of the cavity known as the 

 coelom. This kind of mesoderm is called mesothelium. In some cases, 

 as in Amphioxus, the starfish, Sagitta, and the Brachiopoda (Figs. 462, 

 438, 430, 427 A), it arises as pouches of the archenteron which separate 



