METAZOA 121 



The mesoderm, whose appearance converts the gastrula into a 

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

 which originate in two different ways, namely: 



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

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

 derm is known as mesenchyme, and is comparable to the cells which 

 invade the jelly of coelenterata. 



(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. 437, 

 413, 405, 402 A), it arises as pouches of the archenteron which separate 

 from the latter, their cavity becoming the coelom and their wall the 

 mesothelium. In other cases it arises as solid outgrowths or layers 

 shed off from the wall of the archenteron, and coelomic cavities 

 afterwards appear in it. This happens, for instance, in the tadpole. 

 In yet other cases a single pole cell or teloblast, as in annelids and 

 molluscs (Fig. 189), or a group of a few cells, as in arthropods, 

 separate, on each side of the embryo, from the rudiment of the endo- 

 derm, and multiply so as to form a band of cells in which coelomic 

 cavities appear. 



In a few phyla (Platyhelminthes, Nemertea, Nematoda, Rotifera) 

 there is no mesothelium. In most phyla, both kinds of mesoderm are 

 formed. 1 Since mesothelium gives rise to mesenchyme, it is often 

 difficult to distinguish between the two, and to decide what part each 

 plays in the formation of the organs; but, broadly speaking, it can be 

 said that the skeletal, vascular, and some muscular tissues arise from 

 mesenchyme, while in a coelomate animal the peritoneum and the 

 organs derived from it — gonads (ovaries and testes), mesodermal 

 kidneys, etc. — and the principal muscles, arise from the mesothelium. 



After giving rise to mesoderm, the archenteron becomes the rudi- 

 ment of the alimentary canal. Except in Platyhelminthes, the blastO;:^ 

 pore is in various ways replaced by two openings,^ so that(lt)has 

 both mouth and anus. Its wall, the endoderm, forms the Tming 

 of the alimentary canal, except in those regions, known as /ore ^m^ or 

 stomodaeum and hind gut or proctodaeum, which are formed by a 

 tucking-in of the ectoderm at the mouth and anus. The endoderm 

 also gives rise to the various diverticula of the mid gut, such as the 

 liver and other digestive glands, the lungs of vertebrata, etc. 



The ectoderm gives rise to the epidermis (epithelium which covers 

 the body) to certam glands, to the excretory organs known asnephridia, 



^ Chaetognatha have no mesenchyme, and it is scanty in the lower 

 Chordata. 



2 The most primitive way is probably that of Peripatus (p. 283), in which 

 the middle of the blastopore closes and the ends become mouth and anus. 



