Is an earthworm closely related to a fly? Not 

 really, but the two phyla they represent, Annelida and 

 Arthropoda, are, as phyla go, closely related. Both 

 phyla have true segmentation. Also, many anthro- 

 pods, except for their legs, are worm-like. This is 

 especially true of certain insect larvae, for example, 

 the familiar caterpillars of butterflies and moths. 



The final development of a group of ancestral 

 Schizocoela, prior to its later evolution into distinct 

 annelids and arthropods was true segmentation. 

 Moreover, true segmentation is typical of only three 

 animal phyla; the annelids, the arthropods, and the 

 chordates. However, chordate segmentation was in- 

 dependently acquired because it evolved in animals 

 that already were definite eucoelomates. 



SEGMENTED, WORM-LIKE SCHIZOCOELA: 

 ANNELIDA 



We shall now discuss the adaptations of another 

 worm-like group, the annelids, that enabled them to 

 attain greater size and complexity. Again, these are 

 bilateral Eumetazoa that develop from three germ 

 layers. The true body cavity, or coelom, is the only 

 body cavity. It occupies a large part of the body 

 space. Since it is a cavity in the mesoderm, all organs 

 are surrounded by mesoderm and none lie free in the 

 cavity. These animals have solved the problem of 



growth by means of true segmentation, or metamerism. 

 The body is essentially a series of like boxes, called 

 somites, with partitions between them. Within these 

 somites, various organs are repeated serially — the 

 nephridia-type excretory organs, nerve ganglia, and 

 locomotor organs consisting basically of stiff bristles, 

 or setae. The digestive tract is not segmental and 

 shows clearly defined regions for crushing the food; 

 for storing it; for digesting, absorbing, and egesting it. 

 Needless to say, the digestive tract is complete. The 

 circulatory system is also complete, the blood being 

 entirely contained within blood vessels, and respira- 

 tion occurs through the body wall or specialized folds 

 of its surface. The pumping vessel is dorsal and a 

 main collecting vessel is ventral. The dorsal vessel 

 serves the function of a true heart in that it is a pump 

 with valves. The so-called "hearts," which connect 

 dorsal and ventral vessels, act as a pressure-regulating 

 mechanism, receiving blood in spurts from the dorsal 

 vessel, and then contracting to force the blood under 

 a steady pressure into the ventral vessel. The blood 

 is red, owing to the presence of hemoglobin, but the 

 pigments are usually dissolved in the plasma and the 

 circulatory cells are colorless. The ladder type of 

 nervous system is composed of a pair of head ganglia 

 ("brain") and connectives to a solid, double, mid- 

 ventral nerve cord extending the length of the body. 

 The nerve cord has a ganglion and pairs of lateral 

 nerves in each segment. Reproduction may be 



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