III. Fundamental Form, etc. 41 



sucli animals exMbit a radial symmetry. Their individual 

 organs must naturally either have a radiate structure also, or be 

 equal in number to the antimeres. 



Most Metozoa are, however, bilateral in plan: the body is 

 ■capable of division into two nearly equal parts, which are similar to one 

 another, looking-glass-wise, but are not congruent; the parts of the body 

 are here symmetrically disposed in relation to a median plane. 



One of these types governs the structure of all Metazoa. Neither 

 is, of course, ever carried out with quite mathematical precision, 

 but in many cases it is clearly manifested throughout the whole body. 

 In other cases, variations are obvious ; in many Echinoderms, for 

 instance, the body can be cut into five parts, which, indeed, shew 

 many points of accordance, but are yet far from identical. This 

 is true also for many bilateral animals : amongst Vertebrates, for 

 •example, most of the organs are usually symmetrical in form and 

 arrangement, but as a rule, the greater part of the digestive tract 

 in the adult is an exception, though in the youngest embryos it 

 is symmetrical ; still more aberrant is the condition in others, for 

 the symmetrical type is clear only in certain parts of the body, 

 whilst in most regions it is difficult to make out (Gastropoda). 



In some groups the bilaterally symmetrical body is divided 

 into a series of similar segments (metameres) . The segmental 

 arrangement of many Chfetopoda is representative of the simplest 

 form of this metamerism. The body is composed of a number of 

 segments or rings, which, from the first to the last, are essentially 

 alike, both externally and internally : each ring contains a pair of 

 •excretory organs, a pair of nerve ganglia, is provided with a pair of 

 parapodia, etc. Amongst other metameric animals, e.g., the Arthro- 

 poda, the segments are not as a rule so uniform in structure. 



Amongst bilateral animals, a dorsal and a ventral side may be 

 •distinguished, and further, an anterior and a posterior end. The 

 ventral side is that part of the body which is turned downwards 

 when the animal is moving ; the other is the dorsal side : or more' 

 exactly, that side of the body which, in the majority of animals 

 belonging to a large natural group, is turned downwards, is known in 

 all the members as the ventral surface; the overlying surface is 

 dorsal. In all Gastropods, for example, the ventral side is that which 

 is provided with the so-called foot, and this holds good for all the 

 group, even when, as in pelagic forms, the foot is turned upwards. 

 The anterior end is usually characterised by the presence of 

 the mouth, of certain sense organs, and of a large portion of the 

 central nervous system (the brain) ; and by being usually directed 

 forwards during locomotion : it is often marked off from the rest of the 

 body, or in some way developed in contrast to it, and then it is known 

 as the head. The opposite, posterior end is often also peculiar in 

 construction, e.g., it is thinner than the rest of the body, or specially 

 muscular, and then is called the tail. 



