288 SIDNEY F. HAEMER. 



groups of parietal muscles {p. rii.) are seen, of whicli the distal 

 group {p. m.') originates from the basal wall, and the rest 

 from the vertical walls, close to their junction with the basal 

 wall. 



These arrangements are moderately constant in all the 

 zooecia. The distal group of parietal muscles is always strong, 

 and is commonly stronger than the others. The number of 

 groups does not vary much, but it is important to notice that 

 the distal group may (rarely) originate from the base of a 

 vertical wall, and conversely that the other parietal muscles 

 may originate from the basal wall. The distal parietal muscles 

 thus clearly belong to the same series as the rest. 



The mode of action of the parietal muscles has been well 

 described by Nitsche (1871, p. 426), who showed that the 

 pressure on the fluid of the body-cavity due to their contrac- 

 tion was the main cause of the protrusion of the polypide. A 

 more detailed account of the same process is given by Calvet 

 (1900, p. 63). 



F. pisciformis has no distinct divaricator muscles. The 

 opening of the operculum is probably largely due to the fluid- 

 pressure brought about by the conti-action of the parietal 

 muscles generally ; but it seems to me highly probable that 

 the distal group of these muscles, by pulling on the membrane 

 with which the base of the operculum is immediately 

 continuous, may have a special effect in opening the operculum. 

 1 regard the distal parietal muscles as the starting point from 

 which the divaricator muscles of Euthyris and its allies 

 have been derived. 



Flustra papyrea. Pall. — This species is very similar to 

 F. pisciformis in essential respects. The more noteworthy 

 differences are (1) the parietal muscles are moi'e numerous, 

 usually numbering about six to seven groups on each side j^ 

 (2) while each of the ordinary groups consists of not more 

 than two or three fibres, the distal group is a much broader 

 and more dehnite muscular band, composed of a considerably 

 larger number of fibres; (o) the distal as well as the other 

 ' Tliuy are sliowu in my paper (1892), pi. ii. 



