METAZOA 143 



muscle contains two sets of fibres, one of which by rapid contraction 

 brings an organ into a certain posture, in which it is held by tonic 

 contraction of the other (the *' catch" fibres). 



Most of the energy expended by an animal is liberated in its con- 

 tractile tissues. It is obtained, normally from carbohydrates, by a 

 process which, as we have seen (p. 140), is at first anaerobic and then 

 aerobic (see also p. 662). 



The rudiments of the gonads may be situated either in ectoderm, 

 endoderm, or mesoderm. In Coelomata they always arise in meso- 

 thelium. However, since they are often recognizable as early in 

 development as the layers, and the cells of which they are composed 

 may migrate from one layer to another, and they do not form tissues, 

 they are best regarded as an independent entity. 



The body constituted by the elements described above has usually 

 a bilateral symmetry, though this is rarely exhibited completely by 

 all the systems. In the Coelenterata and Echinodermata, however, 

 there is a radial symmetry. It is interesting to find that a sessile life, 

 for which such symmetry seems particularly advantageous, is cha- 

 racteristic of the Coelenterata, and was probably adopted by the 

 ancestors of all the Echinodermata. The terms ventral and dorsal, 

 which belong by right respectively to those aspects of a bilateral 

 animal which are normally turned to and from the ground or sub- 

 stratum, are sometimes conveniently applied to a pair of structures 

 by which two sides may be distinguished in the body of an animal 

 whose symmetry is predominantly radial. They should, however, 

 never be applied to the oral and aboral aspects of such an animal. 



Meristic repetition of organs of the body is common in Metazoa. It 

 may, as in parts of the body of annelids, affect practically all systems, 

 so that there is a complete segmentation of the body into similar 

 somites, or may be confined to certain organs. In the latter case it is 

 important to distinguish between {a) the repetition of single organs 

 in an unsegmented animal, as the ctenidia and shell plates are in- 

 dependently repeated in the mollusc Chiton, and {b) the condition, 

 presented for instance by the Vertebrata and by much of the body 

 of many arthropods, in which a formerly more complete segmentation 

 now affects only some of the systems to which it at one time extended. 

 The student should beware of thinking that the segmentation of all 

 animals which present the phenomenon is derived from that of a 

 common ancestor. The strobilation of the Cestoda in preparation 

 for the detachment of reproductive units is a very different matter 

 from the segmentation of the Annelida, and that again is far from 

 being, as is sometimes assumed, certainly the same thing as the seg- 

 mentation of the Vertebrata. 



The anterior end of a bilateral animal is the site of the principal 



