THE MORPHOLOGY OF THE CHEILOSTOM ATA . 287 



walls the calcareous matter is in two layers separated by a 

 chitinous lamella/ as described by Nitsche (1871, pp. 421, 

 455) . Each, zooecium tlius has its own calcareous lateral walls, 

 distinct from those of its neighbours. In two regions, 

 respectively proximal and distal to the broadest part of the 

 zooecium, the lateral wall is thickened at its frontal edge. 

 At tlie proximal narrow end the calcareous Avail becomes 

 deficient, and the terminal partition wall, which is thicker 

 than any of the others, belongs to the proximal zooecium of 

 the two which it divides. This accords with Levinsen's 

 statement (1891, p. 251) that in many Cheilostomes the 

 terminal wall is single, while the lateral walls are double, so 

 that the longitudinal rows of zocecia can be isolated from one 

 another by boiliug with caustic potash. The frontal surface 

 is entirely membranous ; the operculum is merely a part of 

 this membrane, and has no basal sclerite. When the oper- 

 culum is open (fig. 3) it is seen to have a vertical flange, 

 produced into lateral points, as in Euthyris. A similar 

 arrangement is described by Nitsche - in Membranipora 

 membranacea. The occlusor muscles [occl.) are inserted 

 into these points, and originate, as in Euthyroides episco- 

 palis, from the basal wall. A pair of strong parieto-vaginal 

 muscles {'p. v. m.) pass from the tentacle sheath to the basal 

 wall, and a smaller pair (p. v. 7n/) connect the tentacle sheath 

 with the frontal wall ; but none pass to the vertical walls 

 (cf. Calvet, 1900, p. 199). A pair of strong parieto-dia- 

 phragmatic muscles (p. d.) spring from the basal wall, just 

 internal to the origin of the occlusor muscles, and are inserted 

 into the diaphragm or vestibule. The retractor muscles (r. ?»..) 

 of the polypide have their origin in one of the corners of the 

 fish-tail-like proximal end of the zooecium. About four 



' 111 a recent paper by Scliuiz ('Arcli. f. Naturg.,' Ixvii, Bd. i, Heft 2, 

 1901, p. 118, pi. vi, fig. i) tills cliitinous lamella is figured as a much tliickcr 

 layer than anything 1 have ever seen. The species investigated by Scimlz was 

 M. " iiieinbranacea " ( := M. nionostachy s, Bii«k ; of. Levinsen [1891], 

 p. -277). 



■ 1871, p. 422. 



