CHAPTER IV 



THE SUBKINGDOM METAZOA 



The fundamental difference in histology which distinguishes the 

 Metazoa from the Protozoa has already been described in Chapter ii. 

 Something must here be said concerning other features of the 

 anatomy of the Metazoa. 



The simplest type of bodily architecture in this subkingdom is that 

 with which the student is familiar in Hydra, where the body consists 

 of a sac with one opening, and with the wall composed of two 

 cellular layers and a layer of secreted jelly between them. The outer 

 layer of cells is the ectoderm, the inner the endoderm. In the phylum 

 to which Hydra belongs, the Coelenterata, the body is always of this 

 type, whatever form the sac or its layers may assume, though the 

 jelly may contain cells, of various kinds and sometimes plentiful, 

 which have migrated into it from the ectoderm or endoderm. 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 coelente rates, but mesoderm is more plentiful than 1 

 the cells in the jelly generally are, it contains important organs and 

 usually definite systems of spaces (see p. 122), 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 different animals may be 

 extraordinarily unlike, and, especially when the animal is developed 

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

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

 development of Amphioxus or in that of a starfish (Fig. 413), 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. / 



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