PROMORPHOLOGT. 



137 



Fig. 89.— Cross-section of a fish passing througli the fore limits. DV^ sagittal axis; 

 liL^ transverse axis; a, dorsal a(ji'ta ; c, body cavity: c/. gut; cl\, notocliord ; y. 

 shoulder-girdle; h, lieart ; m, muscles; ?i, anterior end of the kidneys; p, peri- 

 cardium ; oij, neural arch ; lih^ haemal arch ; r, spinal cord. 



Antimeres and Metameres. — The S3'mmetrical parts of an 

 animal are called antimeref:; each antimere has organs which occur 

 likewise in its adjacent antimere. The right arm of man corre- 

 sponds to the left, the right eye to the left, etc; the same orgaits 

 are repeated in the direction of the transverse axis. Frequently, 

 however, the repetition of organs occurs not only in tlie direction 

 of the transverse axis, but also in tlie direction of tlie long axis. 

 Thns the hody is made np not only of symmetrical parts, the 

 antimeres, hut also of similar parts placed one behind the other, 

 the ■iiietdnieres. 



Internal and External Metamerism. — ^Metamerism or segmen- 

 falion is spoken of when the hody of an animal consists of 

 numerous segments or metameres (consult fig. 50). Very often 

 it is recognizable externally — when, for instance, the limits of the 

 segments are marked on the surface by constrictions (arthropods 

 and annelids). But this external metmnerisin may Ije entirely 

 lackiTtg, and the metamerism find expression only iiiternullij in 

 the serial succession of organs, in metameric or segmental arrange- 

 ment. Man, for example, is segmented only internally; in his 

 skeleton there are numerous similar parts, the vertebras, which 



