THE MAIN LINES OF ANIMAL EVOLUTION 



tural units of the body are repeated serially along the length of the body, 

 and the segments may be marked externally. A well-developed coelom 

 separates the digestive tract from the muscular body wall. There is an 

 advanced nervous system based upon a pair of cerebral ganglia and a pair 

 of ventral cords on which ganglia are located in each segment of the body. 

 Whenever there is a larval stage, it is a trochophore. Most annelids have 

 a thin cuticle, and the typical species have segmentally arranged chiti- 

 nous bristles called chaetae. 



The phylum Annelida is best typified by its most primitive class, the 

 Polychaeta. These take their name from their numerous chaetae, which 

 arise from limb-like lobes, the parapodia, on each body segment. There is 

 typically a distinct head which may have appendages. The sexes are sep- 

 arate, and fertilization is external. There is a trochophore larva. The poly- 

 chaetes are adapted to a wide range of habitats, including pelagic and 

 bottom-dwelling, surface-crawling and burrowing, and permanent tube- 

 dwelling, and they show very striking morphological adaptations. In num- 

 bers of species, they undoubtedly exceed the remaining classes combined. 

 Yet they are less well known because they are almost exclusively marine. 



Better known, but far less typical of the phylum, is the class Oligo- 

 chaeta, including the earthworms and many small freshwater annelids. 

 The class appears to have been formed from the polychaetes by a process 

 of reduction and simplification. The head region is much reduced and 

 never includes appendages. The chaetae are reduced both in size and in 

 numbers, and they are no longer set on parapodia. Oligochaetes are all 

 hermaphroditic. Development is direct, there being no larval stages. The 

 embryos develop in a cocoon, an adaptation to terrestrial life. The class 

 is much more uniform than is the Polychaeta. 



The class Archiannelida is a small group of extremely simplified marine 

 annelids. These are all small worms which generally lack parapodia and 

 chaetae. They are ciliated, like the young of polychaetes. Their nerve 

 cords retain the primitive connection with the epidermis. The coelom is 

 only slightly developed. The Archiannelida were named with the intention 

 of inferring that this was the most primitive class of the phylum and the 

 probable progenitor of the other classes. But embryological and morpho- 

 logical studies have led to the conclusion that this class is not truly primi- 

 tive, but rather has been derived from the Polychaeta by extreme simpli- 

 fication. 



The class Hirudinea, or leeches, appear to have been derived from 

 oligochaete ancestors, for they share many characters with that class. But 

 all leeches are ectoparasites, and they are extensively modified for that 

 mode of life. 



Phylum Arthropoda. The protostome series is climaxed by the great 

 phylum Arthropoda, the most successful of all groups of animals if num- 

 bers of species be an indication of success, for about three fourths of all 

 species of animals are arthropods. This is also the most varied of phyla, 

 for there are arthropods adapted to every imaginable habitat from the 

 abyssal depths of the ocean (crabs and pycnogonids) to aerial heights 

 (insects). Perhaps the key to arthropod evolution is their early develop- 



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