THE MORPHOLOGY OF THE OHEILOSTOMATA. 



339 



with Gregory (p. 222) that Catenicella, which he places 

 nevertheless in the Cellularina, has but little to do with the 

 other Cellularines. I cannot accept the divisions Schizothj^- 

 riata (forms with a sinus or a median pore) and Holothyriata 

 (other Escharine genera), because it seems to me that there 

 is no essential difference between the Schizothynate 

 Schizoporella and the Holothyriate Lepralia. Both 

 these genera probably require a good deal of rearrangement 

 and subdivision; but among recent species there are 

 certainly cases in which it is an open question to which 

 genus they should be referred. It would perhaps have been 

 better to institute a new sub-order for the Microporelloid 

 genera. The question is complicated by the fact that all 

 "median" pores are not homologous structures. It is a 

 well-known fact, for instance, that in some cases (e. g. 

 Adeonella) the median pore is simply a peristomial 

 opening. 



VII. Summary op the more Important Results. 



(1) A large number of Cheilostomata are provided with a 

 "compensation-sac," a structure described by Jullien, which 

 opens at the proximal border of the operculum or by a 

 " median pore." Muscles which usually originate from the 

 lateral walls of the zocBcium are inserted into the floor of the 

 compensation-sac, which they dilate by their contraction, 

 thereby helping to force out the polypide. The constant 

 change of water in the compensation-sac probably has a 

 respiratory importance. The sac is frequently found in 

 zooecia whose polypides have undergone histolysis. 



(2) The floor of the compensation-sac corresponds in whole 

 or in part with the membranous frontal surface (for which 

 the name "frontal membrane" is suggested) of a ]\Iem- 

 branipora; and its muscles are homologous with the 

 parietal muscles of the same genus. The compensation-sac 

 appears to have been evolved in more than one way. 



(3) In (Jril)rilini(la3 it is well known that a calcareous wall 

 (for which and for other calcareous frontal walls the name 



