244 SELIG HECHT 
but stand out expanded on the siphon in the form of thin lappets 
(fig. 5). 
Under special circumstances it is possible to secure a local 
contraction of the region near an individual lappet. Ordinarily, 
however, the entire siphon rim shuts as a unit. This closure is 
Fig. 5 Sketch of living, expanded Ascidia, to show the cheeks on the right 
side and the protruding lappets on the siphon rims. 
conditioned by the presence of well-defined ridges and folds in 
the test, along which the contraction takes place. An end-on view 
of a nearly closed oral siphon (fig. 6) shows that the alternation 
of folds and ridges depends on a surprisingly accurate pattern, 
which involves thick and thin portions of the supporting test. 
