THE MOST PRIMITIVE METAZOA 51 



the various uses of the expressions " primitive; simple; advanced; 

 radial symmetry and bilateral symmetry," since these terms will 

 frequently be used in the following discussion. 



Primitive and simple 



There are two terms that must be distinguished and used 

 carefully. The first term is " simple." If an animal has a 

 morphological structure made up from a few basic units, then such 

 an animal can be regarded as having a simple structure. Other 

 animals may have many different units arranged in a variety of 

 patterns ; they can then be regarded as having a complex structure. 

 These two groups, the simple and the complex, can also be 

 described as having a low level of complexity or a high level of 

 complexity. 



The second term is " primitive." This means that of two 

 structures or conditions, one arose some time before the other. The 

 concept of "time of origin" is the critical point in determining 

 whether a structure is primitive or not. Because an animal has a 

 simple morphological pattern it does not mean that it had an 

 early evolutionary origin and therefore is in a primitive condition. 



It is perhaps unfortunate that during most courses of Zoology 

 the students are taken from the Protozoa to the Primates and 

 shown the way in which the complexity of structure increases. 

 Quite often the student becomes puzzled when he deals with the 

 Mollusca. Should they come before the annelids, between the 

 annelids and the arthropods, or after the arthropods? It is clear 

 that this problem confuses two issues: firstly the complexity of 

 molluscs in relation to that of the annelids and the arthropods, 

 and secondly the time of origin of the molluscs, i.e. did they arise 

 before or after the annelids? 



The student is usually taught that certain conditions can lead 

 to a simplification of morphological form and that clues other than 

 purely morphological ones must be used to elucidate an animal's 

 phylogenetic position. In particular this holds when we come to 

 deal with parasitic animals. Thus the larval form of Sacculina 

 quite clearly shows the crustacean ancestry of the parasite even 

 though the morphology of the adult is not at all typical of the 

 Crustacea. Though the parasitic habit is usually associated with 

 certain morphological changes, the other specialised ecological 



5— IOE 



