One of the questions of interest in the study of the evolution of spiders is whether the weaving of orb webs evolved only once or several times. About half the 35,000 known kinds of spiders make webs; a third of the web weavers make orb webs. Since most orb weavers belong either to the Araneidae or the Uloboridae families, the origin of the orb web can be determined only by ascertaining whether the families are related. Recent taxonomic analysis of individuals from both families indicates that the families evolved from different ancestors, thereby contradicting Wiehle's theory. This theory postulates that the families must be related, based on the assumption that complex behavior, such as web building, could evolve only once. According to Kullman, web structure is the only characteristic that suggests a relationship between families. The families differ in appearance, structure of body hair, and arrangement of eyes. Only Uloborids lack venom glands. Further identification and study of characteristic features will undoubtedly answer the question of the evolution of the orb web.

17. The primary purpose of the passage is to

(A) settle the question of whether orb webs evolved once or more than once
(B) describe scientific speculation concerning an issue related to the evolution of orb webs
(C) analyze the differences between the characteristic features of spiders in the Araneidae and Uloboridae families
(D) question the methods used by earlier investigators of the habits of spiders
(E) demonstrate that Araneidae spiders are not related to Uloboridae spiders



18. It can be inferred from the passage that all orb-weaving spiders belong to types of spiders that

(A) lack venom glands
(B) are included either in the Uloboridae or Araneidae families
(C) share few characteristic features with other spider types
(D) comprise less than a third of all known types of spiders
(E) are more recently evolved than other types of spiders



19. According to the passage, members of the Araneidae family can be distinguished from members of the Uloboridae family by all of the following EXCEPT

(A) the presence of venom glands
(B) the type of web they spin
(C) the structure of their body hair
(D) the arrangement of their eyes
(E) their appearance



20. Which of the following statements, if true, most weakens Wiehle's theory that complex behavior could evolve only once?

(A) Horses, introduced to the New World by the Spaniards, thrived under diverse climatic conditions.
(B) Plants of the Palrnaceae family, descendants of a common ancestor, evolved unique seed forms even though the plants occupy similar habitats throughout the world.
(C) All mammals are descended from a small, rodent-like animal whose physical characteristics in some form are found in all its descendants.
(D) Plants in the Cactaceae and Euphorbiaceae families, although they often look alike and have developed similar mechanisms to meet the rigors of the desert, evolved independently.
(E) The Cuban anole, which was recently introduced in the Florida wilds, is quickly replacing the native Florida chameleon because the anole has no competitors.