320 SIDNEY F. HARMKR. 



At its origin this muscle is often difficult to distinguisli 

 from tlie depressor, but its tendon takes a different direction 

 and is inserted into one of the ends of the basal sclerite of 

 the operculum, or perhaps into the frontal membrane imme- 

 diately adjacent to the basal sclerite. 



I regard the divaricator and depressor muscles as a special 

 differentiation of the parietal muscles of other Clieilostomes, 

 and they are probably together the representatives of the 

 depressor muscles of Micropora (Kgs. 57,58), which occupy 

 special cavities that may be compared with the lateral 

 recesses of Steganoporella.' 



In the A-zooccia the distal occlusors seem to be completely 

 absent. The zocecium is more like that of an ordinary 

 Clieilostome, with a single pair of occlusor muscles. 



In view of the possibility that the B-zoa3cia of Stegano- 

 porella may represent avicularia,^ it is worth mentioning that 

 in the gigantic avicularia of Flustra abyssicola (Davis 

 Sti-aits) I have noticed that the occlusors are in two groups. 

 Of these the distal group consists of an enormous mass of 

 muscle which fills up more than the distal half of the 

 avicularian zooecium, and its hbres are inserted into a broad 

 median tendon. The proximal occlusors are paired ; their 

 tendons are at a deeper level than that of the distal occlusor, 

 and they converge towards the base of the operculum. 

 Jullien (1888, 4, pi. xii, fig. 3) indicates the occurrence of two 

 sets of occlusors in the avicularium of Beauia magel- 

 1 a n i c a. 



V. The Pkimary Zocecium or Ancestkula. 



The zooccium which results from the metamorphosis of the 

 larva is frequently found to show characters which dilTer 



' This conclusion is coufinncd by llic (igurc given by Waters (1885, 

 pi. xiv, fig. 42) of the mandible of Oiiycbocella angulosa (? = antiqua), 

 ill wliicli the formation of the hinge appears to be like that found in Stegano- 

 porella, while a fascia shown in the same figure closely resembles that of the 

 distal occlusors of St ega no j) ore 11 a. 



