METAZOA 131 



to the alimentary canal proper but may take place in digestive glands 

 or "livers", as for instance in those of the mussel, the snail, and the 

 crayfish, but not in those of cuttlefishes or vertebrates. It is said that 

 in various bivalve molluscs and in holothurians amoeboid corpuscles 

 pass through the endoderm, take up particles in the gut, digest them, 

 and, returning, distribute the products. The presence of a cuticle in 

 the ectodermal portions of an alimentary canal does not always prevent 

 absorption there (e.g. in the fore gut of some insects). Finally it should 

 be noted that some animals perform a part of their digestion externally 

 to the body J as the starfish by extruding its stomach (p. 636), and 

 various insects, mites, earthworms, etc. by pouring out saliva ; and that 

 in other cases bacterial or protozoan symbionts (pp. 68, iii) play a 

 part in the digestion of food — particularly of celluloses — in the gut. 



The food of all animals contains amino acids, usually as protein, 

 for the manufacture of the proteins needed in the repair and growth 

 of protoplasm. Much amino acid, however, is deaminated^ the car- 

 bonaceous residue being oxidized, together with the carbohydrate 

 and fat which the food usually also contains, for the liberation of 

 energy, and the ammonia excreted in various forms by various organs 

 presently to be mentioned. 



ii. Mesodermal organs. Since mesothelium gives rise to mesen- 

 chyme, it is often difficult to distinguish between the two and to 

 decide what part each plays in the formation of organs ; but broadly 

 speaking it can be said that the connective and endoskeletal, the 

 vascular, and some muscular tissues arise from mesenchyme, while 

 in coelomata the peritoneum and the organs derived from it — gonads 

 (ovaries and testes), mesodermal kidneys, etc. — and the principal 

 muscles arise from mesothelium. 



Within the massive layer of mesoderm, cavities are necessary for 

 sundry purposes. Channels must be provided for the transport of 

 various materials — the products of the digestion of food, the gases of 

 respiration, water, the waste products of metabolism, which are 

 usually eliminated with the excess of water, and the substances known 

 as hormones which are secreted by certain organs as messengers to 

 regulate the activity of others. The germ cells, which are sheltered 

 in this layer, must be given access to the exterior. Often there must 

 also be spaces to give play to movements of the viscera. Such facilities 

 are provided by two systems of cavities, the primary and secondary 

 body cavities^ of which either or both jnay be present. 



{a) The primary body cavity, sometimes known as the haemocoele, 

 is to be regarded, morphologically, as representing that part of the 

 blastocoele which is not obliterated by the mesenchyme cells or by a 

 solid matrix or fibres secreted by them. Its fluid contents, containing 

 free mesenchyme cells ("corpuscles"), are the blood and lymph, and 



