206 HYATT, 



shorter, and the invagination less ; in Pectinatella, where 

 there are no free cell-walls, the invagination is very much 

 less ; and in Cristatella, which has neither free cell-walls 

 or lobes, the invagination is nothing. Even the zooids of 

 the species show the action of this law. Whenever the 

 free portions of the cells are much reduced in length, as 

 in some zooids of variety c of Plumatella vitrea, the 

 number of rows of posterior retentors are less, and the 

 evagination greater, than in other zooids of the same 

 colony, with longer walls to the free portions of the cells. 



The Alcyonelloid varieties of Plumatella Arethusa have 

 only about eight rows of posterior retentors ; while the 

 varieties with distinct cells may have as high as fifteen. 



Thus, with the exception of the fluctuation or .disturb- 

 ance occasioned by the muscular walls in Cristatella, and 

 the increase and decrease of the cosnoacial orifice, which 

 causes the increase and decrease in the number of bands 

 of the anterior retentors, all the characteristics of the 

 evaginatory system are traceable to the decrease in the 

 free portions of the cell- walls. In other words, to the 

 gradual obliteration of the cells and branches. But this, 

 as we have seen previously, is due to the activity of the 

 reproductive function, which steadily increases the num- 

 ber of buds arising from the primary ccencecial cell. 



LOPHOPHOKIC SYSTEM. 

 LOPHOPHORE. 



The outline of the lophophore is an incremental char- 

 acter, owing its increment to the growth of the arms in 

 Plumatella. 



ARMS. 

 The length of the arms is the key-note of the changes 



v ~ 



in this system, and we shall find, as we proceed, that the 

 increment of this characteristic determines the increment 

 or decrement of nearly every variable characteristic. 



Tips divergent tips twisted outwards. These two 

 characters go together. They are decremental ; and the 

 decrement is due to the retention of embryonic characters 

 in the adults of the higher genera. 



