14 Dr Harmer on the Structure and Classification 



The operculum is merely in contact with the calcareous wall, 

 and retains its original relations in being continuous with the 

 membranous floor of the compensation-sac. Numerous muscles 

 pass from the vertical calcareous walls of the zooecium to the 

 floor of the sac. It may safely be concluded that the contraction 

 of these muscles will dilate the sac, thereby introducing water 

 into it from the outside and exercising a pressure on the fluid of 

 the body-cavity, resulting in the protrusion of the polypide. The 

 muscles which produce this effect are thus, in their function as in 

 their morphological nature, to be regarded as parietal muscles ; 

 and their relation to the compensation-sac is an important link in 

 the chain of evidence tending to show that the calcareous " front 

 wall" is derivable from an arrangement similar to that of 

 Cribrilinidae. The front wall may be developed in two very 

 different ways : — 



(a) In Umbonula verrucosa, it grows over the aperture at a 

 higher level, as a continuous overarching lamina, which commences 

 proximally and laterally ; the space which it covers always remain- 

 ing widely open to the exterior. A Membranipora-\ike opercular 

 wall is present in the young zooecium, and the parietal muscles 

 develop in situ. This arrangement is in no way modified during 

 the later development, and the operculum still retains its primitive 

 condition of having no basal sclerite. So far as I can judge from 

 dry material, calcification proceeds in the same manner in certain 

 species of Porella, Mucronella and Escharoides ; which are accord- 

 ingly to be regarded as related to Umbonula. 



(6) In Lepralia pallasiana, Schizoporella linearis, Euthyris 

 obtecta, Catenicella cornuta, and others, a different mode of 

 development is followed. The front wall appears at first sight to 

 result from the direct calcification of a Membranipora-\ike 

 membranous wall which is usually distinctly visible in the young 

 zooecium. The compensation-sac is at first not present, but 

 develops from a mass of cells beneath the proximal edge of the 

 operculum. These cells soon arrange themselves round a cavity, 

 which appears semicircular when seen from above, the diameter 

 of the semicircle coinciding with the proximal edge of the oper- 

 culum, and the arc curving on the proximal side of the base. 

 Numerous muscle-fibres radiate from the walls of this sac to the 

 more proximal parts of the lateral walls of the zooecium. The 

 sac rapidly grows in a proximal direction until it underlies the 

 whole or the greater part of the calcareous front wall. The 

 muscles which at first radiated from it are now arranged as two 

 lateral series of parietal muscles. The identity of arrangement 

 of these muscles and those of other Cheilostomes suggests that 

 the phylogenetic origin of the calcareous wall has here been 

 modified in ontogeny, in such a way that the development of the 



