OBSERVATIONS ON POLYZOA. 205 



in the size of the cavity of the ccencecium do not appear 

 to affect materially their intimate structure. 



RECAPITULATION . 



We have seen: (1) that the increase in the size of 

 the Alimentary canal was due, apparently, to the enlarge- 

 ment of the ccencecium, occasioned by the obliteration of 

 the cell-walls in Pectinatella, and its decrease to the sub- 

 sequent redivision of the ccencecium into cells in Crista- 

 tella ; (2) that this increment and decrement was confined 

 to the stomach and intestine, the oesophagus steadily de- 

 creasing in the proportions of its length to its breadth 

 throughout; (3) that the passage of the whole alimentary 

 canal out of the ccencecium, when the polypide is evagin- 

 ated, is due to the increasing length of the evaginable 

 tube, and therefore referable to the increasing evagination 

 of the polypide ; (4) that the increasing size and diffusion 

 of the anterior bases and posterior filaments of the retrac- 

 tors, was also due to the increasing evagination of the 

 tube ; (5) that the increase and decrease in the number of 

 the anterior retentors was due to the increase and de- 

 crease in the size of the ccencecial orifice ; and (6) that the 

 approximation of the anterior retentors to the orifice, the 

 decrease in the breadth of the sphincter, and the increas- 

 ing length of the evaginable endocyst or tube, are due to 

 the steadily decreasing breadth of the invaginated fold ; 

 (7) that this last is due to the decrease in the number of 

 rows of posterior retentors, which decrease may in turn 

 be traced* to the decreasing length of the free part of the 

 cell-wall. Or, we may say : (1) that the free portions of 

 the cell-wall becoming shorter, destroy the lower rows of 

 posterior retentors; and thus (2) portions of the breadth 

 of the invaginated fold are set free ; (3) which are added 

 to the length of the evaginable tube, and therefore the 

 increase of the evagination is due to the decrease in length 

 and final obliteration of the cell-walls. 



Thus, in Fredericella, where the walls are longest, the 

 invagination is greatest ; in Plumatella, the walls are 



* Vide description of "Muscular System." 



