32 



The Compensation-Sac. 



The compensation-sac was first noticed by Jul lien', hut llie descrip- 

 tion which tliis writer f^ives of this iniporlaul organ is not very detailed, and 

 this might be one of the reasons why Jul lien's discovery has either been quite 

 overlooked or received with distrust by all later writers, until Harmer- redis- 

 covered this organ and gave a fuller account of it. While I do not agree with 

 Harmer in his view of the structure and development of the compensation-sac 

 in Discopora verrucosa and the forms grouped with this species, my investigations 

 agree otherwise with the general view he gives of this organ. It is a large thin- 

 walled sac, which in the Cheilostomata provided with an arched calcified frontal 

 wall (Ascophora mihi) lies immediately under this in the whole or the largest part of 

 its extent and oj)ens outwards either immediately jjroximally to the o])erculum, or 

 occasionally further back, through an unpaired median opening, an ascopore« (Micro- 

 porella, Inversiiila, Tiihucellarid, Onchoporella, Haplopoma, Adeona). Its inner wall, 

 which is attached distally to the proximal edge of the operculum, is on each side 

 furnished with a row of muscular bundles, which in arrangement, grouping and 

 attachment to the inner surface of the calcified lateral walls, correspond perfectly 

 with the parietal muscles in the Malacostega, and there is no doubt that they have 

 the same signification as the.se, because their contraction will extend the sac, thus 

 causing it to be filled with water through its external opening with the final result 

 that the polypide is extended. The observations made with regard to the first 

 l)eginnings of this sac scarcely leave any doubt, that as a rule it is formed as 

 an invagination of the original membranous frontal wall of the zooecium, whether 

 the wall later on retains its membranous condition as in all groups furnished 

 with a cryptocyst, or later becomes calcified as in Hippotlioulde and Cdleiutriklitc. 

 The first trace generally appears rather late, either distally to the operculum or 

 to the median pore, and from there it gradually .spreads over the rest of the 

 frontal wall. A somewhat ditfex-ent mode of formation is found however in Disco- 

 pora verrucosa and related forms. While Harmer"' looks upou the compensation- 

 sac as a true sac provided both with an inner and an outer membranous wall in 

 all other forms examined by him, he has quite a different view of the corre- 

 sponding formation in the above-mentioned forms. He states namely, that in 

 these the membranous frontal wall provided with parietal muscles, which is 

 seen on the (piilc young zoo'cia, later becomes covered by a fold, springing from 

 the proximal and lateral margins, the inner lamella of which is calcareous and 

 the superficial layer membranous, and the compensation-sac formed by Ibis 



' 45 a. p. 67—68; ' 18 and 19: ' lit, p. 293—297. 



