MUSEUM OF COMPARATIVE ZOOLOGY. 33 



seen any such circular groove surrounding the branch as he figures. In 

 all cases the two layers of the body wall form a circular fold, in which, 

 however, there is never, even at the earliest stages, a space between the 

 ectodermal layers, nor any infolding of the cuticula as Korotneff ('75, p. 

 369), according to Hoyer's rather incomplete abstract, maintains (Plate 

 V. Fig. 47). When the circular fold has advanced until only a small pore 

 remains, by which the cavities of the older and younger individuals are 

 kept in communication, the mesodermal cells at the angle of the fold 

 begin to undergo a metamorphosis both in form and histological charac- 

 ter. In the first place they become much elongated and extremely 

 attenuated, passing from one surface of the septum to the other, and 

 forming the lips of the pore. In the second place their plasma becomes 

 first deeply stainable, and later, in addition, homogeneous and highly 

 refractive. These metamorphosed cells form what may be called the 

 teeth of the plate. They are derived wholly from mesoderm. 



The cells in the upper mesodermal layer next increase rapidly in 

 number and size, and the number of teeth is also augmented (Plate V. 

 Fig. 48). The metamorphosis of the cells extends still farther away 

 from the communication pore, and involves the lower mesodermal layer ; 

 but, apparently, each cell of the latter is metamorphosed only to a 

 slight depth within its cell wall (Fig. 51), whereas in each of the iipper 

 cells the ends which project into the communication pore are modified 

 through and through (Fig. 46). At a later stage (Fig. 49) the meta- 

 morphosed part of the cell seems quite sharph^ cut off from the active 

 part, and the slits between the metamorphosed teeth are considerably 

 reduced. Nevertheless, I believe a transfer of fluids may still occur 

 between them, for even in the adult communication plate one can trace 

 continuous lumina when the cells ai-e by accident torn off from the 

 "teeth" which they have produced. It is important to note that the nu- 

 clei are not destroyed in the cell metamorphosis. Some lie above, others 

 below the pore, and become deeply stainable. The ectodermal layers of 

 the communication plate seci-ete a cuticula between them. This is thin- 

 ner than that of the body wall, and does not extend, of coui'se, to the 

 centre of the communication plate, but ends in a thTckened ring, whose 

 diameter is about one tenth the diameter of the plate, or, absolutely, 

 about 9.4 ii} 



1 Eeichert ('70, p. 2(57) first carefnllv described the Kosettenplate of Cteno- 

 stomes in Zoiibotryon, and the organ in Paludicella must be regarded as homologmis 

 witli it. The central circular hole in the cuticula of Zoiibotryon is from 7 to 10 ;U 

 in diameter, and from one ninth to one seventh that of the entire plate. Similar 



VOL. XXII — xo 1. 3 



