324 



THE ANIMAL KINGDOM 



Figure 16.31. Peripatus, a member of the Onychophora, a "missing link" between 

 the AnneUda and the Arthropoda. (Courtesy Ward's Natural Science Establishment.) 



ner very similar to that in the arthropods and annehds (especially in the 

 heavily yolked eggs of the latter). The coelomic cavities neither form 

 the main body cavity as in the annelids nor disappear as in the arthro- 

 pods, but persist as small cavities associated with annelid-like, metameric 

 nephridia. The adult body cavity is a hemocoel like that of the arthro- 

 pods. The embryonic first segment persists and bears a pair of prean- 

 tennae. The mouth opens on the second segment whose appendages 

 become jaws. The appendages of the third segment lie beside the mouth 

 as oral papillae which can shoot out slime to entangle an enemy. The 

 rest of the paired appendages are short un jointed legs ending in terminal 

 claws. The body covering is a thin, soft cuticle like that of the annelids, 

 but it is beset with numerous small spiracles from which delicate 

 tracheal tubes branch into the body. 



These characteristics are sufficient to differentiate the Onycophora 

 from both the Annelida and the Arthropoda as a separate phylum. Al- 

 though the group is often used as a possible ancestral type for the 

 Arthropoda, linking them to the Annelida, a second look shows that 

 Peripatus, a terrestrial organism itself, forms a rather awkward tie be- 

 tween trilobites and annelids. A view which is now winning acceptance 

 is that the first segmented, pre-annelid, pre-onycophoran, pre-arthropod 

 animals probably radiated into a number of groups, of which three exist 

 today. There is some speculation that the Onycophora may once have 

 been more widespread and may have included a wider variety of forms. 

 This view is supported by the discovery in 1930 of Aysheaia, a Cambrian 

 fossil. The particular rocks in which eleven specimens were found are 

 remarkable for the perfection of their fossils, and contain clear prints 



