THE MOKPHOLOGY OF THE CHEILOSTOMATA. 311 



similar plane of the next pair. The narrow proximal part of 

 each zooecium extends the v/hole length of the snbjacent pair, 

 in such a way that each zooecium originates from the next 

 lower one which looks in the same direction as itself. 



The two lateral horns are not calcified at their tips (fig. 

 62, /). Below the oblique orifice is a triangular region 

 sloping to the median pore (m. p.), which occupies the most 

 projecting part of the zooecium. The middle line of this 

 region is marked by a suture (.v.), on each side of which is an 

 " oculiform " pore. Besides the comTiiunication pore at each 

 end, by which it is connected with its neighbours in the same 

 longitudinal series, each zooecium has a pair of distal com- 

 munication pores placed on the basal side, by which its soft 

 tissues are continuous with those of the narrow proximal 

 half of the next distal ]mir of zooecia, while its own proximal 

 half is similarly provided with a pair of pores (r.p.j which 

 lead to the two zooecia of the subjacent pair. 



The fully developed compensation-sac (fig. 62, c. s.) occu- 

 pies a large proportion of the body-cavity. Fig. 61 repre- 

 sents a compensation-sac which is about half grown, with its 

 paiietal muscles. The sac does not extend to the semi- 

 circular operculum, the straight base-line of which appears 

 to constitute the hinge. It develops in a Lepralioid manner, 

 a small, apparently solid mass of cells appearing beneath the 

 newly formed median pore ; and to this mass the parietal 

 muscles radiate from various parts of the zooecium. Some- 

 what earlier the uncalcified, oblique, terminal wall of the 

 zooecium extends as a triangular point as far as the median 

 pore, so that the sac is probably derived from the proximal 

 part of the orifice. The suggestion has often been made 

 (cf. Hincks, 1880, p. 237) that the Microporellid median pore 

 has been formed by the closure of the sinus of a Schizo- 

 porella-like ancestor. The above-described immature stage 

 of C gracilis appears to point in this direction. 



The occlusor muscles {occl.) are easily seen, but I have 

 found no divaricators. 



