ORGANIC STRUCTURE : MORPHOLOGICAL 



33 



parts. In a general way we may say that the animal body is 

 segmental^ each segment being more or less similar in structure 

 to all the others. In a rough sort of way we distinguish these kinds 

 of symmetry — that is, the various arrangements of segments : 



^^ Upper (dorsal) 



Anter wv \J^\y |^ \ y P osterior^ 



(Head)^\ /"I Vl Vl (tool) 

 ,f ^ 12 3 4 

 Segraervts ^| 



1 



^v' 



\ r.HL 



Lower 



Lower (ventral) 



Fig. 2. — Principal Types of Symmetry of Bodily Segments. 



1. Triple animal symmetry (a fish) : There is differentiation in the anterior-posterior axis, 

 between upper and lower (dorsal and ventral) parts and between the right and left sides. 

 2. Radial symmetry (a starfish) : There are upper and lower parts and parts are arranged 

 radially about an upper-lower axis. 3. Racemose symmetry (hydrozoon or polyzoon) : The 

 " body " (which is to be regarded as a " colony " of attached organisms) branches. The seg- 

 ments (or " individuals " of the colony) may have, each of them, radial, or bilateral (right and 

 left) symmetry. 



The segments are obvious or partially concealed parts of the 

 animal body that are all more or less alike. The polyps, or 

 zooids of a Hydrozoon, or Coral, we regard here as segments. 

 So are the " arms " of a Starfish, the " joints " of an Earthworm 

 or Tapeworm, the imperfectly differentiated " joints " in the body 

 of a lobster, etc. The segmentation is never quite complete in 

 the sense that all the segments are quite alike. The symmetry, 

 also, may be confused and may partake of various types. 



B 



Fig. 3. 



A. Head, or Scolex, of a Tapeworm (or Cestode). B. The anterior part of the same. Here 

 the segments (proglottides) of the worm are very nearly all alike, developing in degree of 

 sexual maturity from anterior to posterior. A detached segment is capable of independent 

 existence. The Scolex shows radial symmetry. 



\oh. Integration of Parts. In no case have we, in the 

 animal kingdom, exact repetition of similar parts, or segments. 

 The latter always differentiate in form and exercise different 

 functions, to some extent. Thus the diversity in the Cestode, as 

 illustrated above. 



