142 THE INVERTEBRATA 



as urea, guanin, and uric acid, which are relatively harmless. In aquatic 

 animals, where plenty of water is available to carry off the excreta 

 rapidly, the latter are principally ammonia compounds. In terrestrial 

 animals it is necessary to expend energy in converting them into 

 substances such as those mentioned above. 



The only triploblastic animals which have a rigid skeleton of great 

 importance are the Echinodermata and Vertebrata, in which it is 

 internal and mesodermal, and the Arthropoda, in which it is a cuticle 

 secreted by the ectoderm and is therefore primarily external, though 

 ingrowths of it may form a kind of internal skeleton to which muscles 

 are attached. 



The muscular system is in coelenterates derived from ectoderm or 

 endoderm, in triploblastica almost entirely from mesoderm, though 

 some muscles of Crustacea arise from ectoderm. In Coelomata fibres 

 from the mesenchyme form only minor muscular structures, such as 

 the walls of blood vessels; the great masses of muscle are mesothelial. 

 In the lower animals the fibres mostly lie parallel to the layers from 

 which they arose, forming a sheet in the gut wall and another, known 

 2iS ihtdermomuscular tube, undtrth.t^'kin. In these sheets there is always 

 a longitudinal and usually also a circular layer; sometimes diagonal 

 fibres are added. The movements which such layers bring about are 

 changes of size and shape of regions of the body or gut wall by the 

 contraction of one set of fibres with relaxation of the other, their 

 action being aided by the changes in turgor which are caused by the 

 compression of the fluids they enclose. When there is a skeleton 

 muscular action is different. The dermomuscular tube is now broken 

 up into muscles which pull upon pieces of the skeleton and so move 

 parts of the body. When the skeleton is internal and the body wall 

 remains flexible, more or less of the dermomuscular sheet remains. 

 It is lost when there is a stiff cuticle. The muscles of limbs are pro- 

 vided by outgrowth from the dermomuscular layer. 



In the lower animals the muscular fibres are usually varieties of the 

 unstriped kind. In other cases, chiefly in higher animals (vertebrates, 

 AmphioxuSy arthropods, part of the adductor muscle in the scallop, 

 etc.) there appears a new type, the striped fibre, more swift and 

 powerful in action but more dependent upon the nervous system; 

 it has lost the power of automatic rhythmical contraction and of 

 retaining without nervous stimuli a certain degree of contraction, 

 known as "tone". Some striped fibres, however, retain one or other 

 of these powers ; thus the fibres of the heart of vertebrates contract 

 automatically and those of the adductor of the claw of crabs and 

 lobsters automatically maintain tone. Tone may also be maintained 

 in striped fibres by the nervous system. In some cases (adductors 

 of the scallop and of crustacean claws, spines of sea-urchins, etc.) a 



