194 The Nature of Biological Diversity 



and the same is true for the two postoral sutures and cytopyges; hut 

 the two vestibules, mouths, and gullets may still he present (Fig. 

 13B) . In this condition, the latter can still he reproduced (Fig. 13B) ; 

 but eventually even the two vestibules, mouths, and gullets become 

 one in the progeny. This is one way in which cells with two oral 



FIG. 13. Two stages (A and B) in the fusion of oral meridians in a line of 

 descent that had the two oral meridians less than 90° apart. 



A. Fission stage: the two new oral apparatuses have formed and separated from 

 the old ones. Each oral meridian has a short preoral suture flanked by right and 

 left kinety fields. Less than halfway to the anterior pole the two preoral sutures 

 meet and proceed to the pole as a single suture flanked by its right and left kinety 

 fields. The two oral meridians have still shorter independent postoral sutures (in 

 the opisthe, of course) before they meet and proceed to the posterior pole as a 

 common suture flanked by a single right and a single left posterior kinety field. 

 There is but one cytopyge; it lies on the common postoral suture. A cleavage line 

 appears to the right of the right oral meridian and (in typical wide form) to the 

 left of the left oral meridian, but it is totally absent between the two oral meridians. 



B. A late fission stage of another animal in the same line of descent. The entire 

 suture line (preoral and postoral) is single. The only residue of doubleness appears 

 in the vestibules, mouths, and gullets, both sets of which have reproduced. 



al, anterior left kinety field; ar, anterior right kinety field; cl, cleavage line; in, 

 mouth and vestibule; pi, posterior left kinety field; pr, posterior right kinety field; 

 py, cytopyge; s, preoral suture; *, area between vestibules where kineties have 

 not cleaved. 



