204 HYATT, 



The number of the bands, however, is dependent upon 

 the width of the orifice, and since this becomes gradually 

 broader up to the maximum in Pectinatella, and then de- 

 creases in Cristatella, they have a corresponding fluctua- 

 tion in the number of the bands. 



Fm' removed from the orifice. The distance of the 

 posterior attachments from the orifice is measured by the 

 breadth of the invaginated fold ; and as this decreases, they 

 of course come nearer to the edge of the opening, finally 

 resting on the edge itself. 



POSTEEIOR EETENTOES. 



The decrement in this series is explained by the pre- 

 ceding paragraph, since, as in the last characteristic, the 

 number of rows is determined by the breadth of the 

 invaginated fold. 



SPHIiSrCTEE MUSCLE. 



This being coextensive with the fold, necessarily de- 

 creases with it. 



INVAGINATED FOLD. 



This decreases in precisely the same proportion as the 

 evaginable endocyst increases. And, in fact, it could not 

 well be otherwise ; since, as previously stated, the increase 

 in the length of the evaginable endocyst or tube is wholly 

 due to the additions made to it by the freed portions of 

 the decreasing invaginated fold. 



EVAGINABLE ENDOCYST. 



The measurements were made from the anterior edge 

 of the invaginated fold to the lophophore. 



Membranes fully developed. The invariability of the 

 cellular structure of the membranes is due, probably, to 

 the similarity of their position, and of the circumstance 

 surrounding them throughout the group. They are not 

 subject to any such changes, as the removal of the ecto- 

 cyst gives room for in the coenoecial endocyst, and changes 



