Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right-or left-handedness. In the case of Lymnaea peregra, studies indicate that a dextral gene is expressed during egg formation; i.e., before egg fertilization, the gene produces a protein, found in the cytoplasm of the egg, that controls the pattern of cell division and thus handedness. In experiments, an injection of cytoplasm from dextral eggs changes the pattern of sinistral eggs, but an injection from sinistral eggs does not influence dextral eggs. One explanation for the differing effects is that all Lymnaea peregra eggs begin left-handed but most switch to being right-handed. Thus, the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.

5. Which of the following would serve as an example of "concomitant structural features" (lines 29 - 30) that might disadvantage a snail of the rarer form?

   (A) A shell and body that are an exact mirror image of a snail of the commoner form

   (B) A smaller population of the snails of the rarer form

   (C) A chip or, fracture in the shell caused by an object falling on it

   (D) A pattern on the shell that better camouflages it

   (E) A smaller shell opening that restricts mobility and ingestion relative to that of a snail of the commoner form

6. The second paragraph of the passage is primarily concerned with offering possible reasons why

   (A) it is unlikely that evolutionary mechanisms could discriminate against sinistral snails

   (B) sinistrality is relatively uncommon among snail species

   (C) dextral and sinistral populations of a snail species tend to intermingle

   (D) a theory based on a developmental mechanism inadequately accounts for the predominance of dextrality across snail species

   (E) dextral snails breed more readily than sinistral snails, even within predominantly sinistral populations

7. In describing the "evolutionary mechanism" (line 40), the author mentions which of the following?

   (A) The favorable conditions for nurturing new offspring

   (B) The variable environmental conditions that affect survival of adult snails

   (C) The availability of potential mates for breeding

   (D) The structural identity of offspring to parents of the same hand

   (E) The frequency of unions between snails of different species

8. According to the passage, which of the following is true of Lymnaea peregra?

   (A) Handedness within the species was at one time equally distributed between left and right.

   (B) Under laboratory conditions, dextral eggs from Lymnaea peregra can be artificially induced to develop into sinistral snails.

   (C) Broods of Lymnaea peregra are, without variation, exclusively sinistral or dextral.

   (D) Handedness in Lymnaea peregra offspring is determined by only one of the parents.

   (E) Geographic factors have played a larger role than has genetics in the evolution of the species.

9. The passage implies that in Lymnaea peregra, there will generally be

   (A) more offspring of the nondominant hand in broods where handedness is determined after, rather than before, fertilization

   (B) a sinistral gene that produces a protein in the cytoplasm of the egg cell

   (C) fewer sinistral offspring in dextral broods than dextral offspring in sinistral broods

   (D) equal numbers of exclusively left- and right-handed broods