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THE CGELOMATE BODY 



BILATERAL SYMMETRY 



All triploblastic animals early acquire bilateral symmetry, so 

 that there is a median plane on either side of which are two 

 halves of the body which are roughly mirror images of each 

 other. In the annehds there is Httle difference between the two 

 sides, but in the molluscs and chordates there is a greater or lesser 

 degree of secondary internal asymmetry, especially noticeable in 

 the coihng of the gut, but often showing itself in blood vessels, 

 gonads or other organs. The external appearance of similar right 

 and left sides is usually maintained much more closely, although 

 differences are sometimes apparent, as in the pincers of the cray- 

 fish and the scrotum of many mammals. The echinoderms com- 

 pletely lose their bilateral symmetry early in embryonic life, and 

 acquire a secondary radial symmetry. 



The existence of bilateral symmetry enables us to distinguish 

 between anterior and posterior ends, between right and left sides, 

 and between dorsal and ventral surfaces. A section cut parallel 

 to the anteroposterior axis and in a vertical plane is called 

 longitudinal, and if it is in the middle line it is sagittal. A hori- 

 zontal section is called frontal, and one which is at right angles 

 to the axis is transverse. The surface of a structure nearer to the 

 axis of symmetry is called medial, that further away is lateral, 

 but these terms are not often used outside human anatomy. The 

 end of a structure such as a limb which is nearer to the body is 

 called proximal, that further away is distal. 



THE CCELOM 



The flatworms and roundworms are solid, except for the gut 

 and excretory and genital canals. All the other triploblastic 

 groups have two large series of spaces in the mesoderm, although 

 sometimes these are reduced. The most obvious of these is the 

 ccelom, often called simply the body cavity. It is sometimes 

 formed from pouches which grow out from the gut while the 

 mesoblast is being formed, sometimes by a separation of meso- 

 derm cells to form a space. Its simplest shape is that of a trough- 

 Uke cavity (in vertical transverse section like a U) partly 

 surrounding the gut, but it is often divided up in various ways. 

 The coelom contains a fluid, in which there may be cells, but it is 

 doubtful if the cavity itself can truthfully be said to have any 



