THE MORPHOLOGY OF THE OHKfLOSTOMATA. 281 



wall that tliis part may not entirely retain its shape in 

 drying. 



Large, oval comraunication-pores, or rosette-plates, occur 

 in the lateral walls, at about the middle of the interval 

 between the frontal and basal surfaces (fig. 14, c.j^-). Each 

 zooecium is normally bounded by two zooecia on each side, 

 and it usually communicates with each of its four lateral 

 neighbours by means of two (rarely three) rosette-plates. 

 In the terminal partition-walls, there is a horizontal row of 

 small pores in place of definite rosette-plates. The vertical 

 walls have no other pores. 



The transparency of this species makes it a favourable 

 one for the study of the compensation- sac. At the growing 

 ends of the branches the frontal surface of the zooecium is at 

 first an nncalcified membrane, in which calcification begins 

 at the proximal end and gradually extends distally. The 

 outline of the operculum becomes apparent before calcification 

 invades its immediate neighbourhood. Shortly before the 

 edge of the calcified frontal wall (fig. 13, x) reaches the 

 region of the future operculum, the part of the uncalcified 

 membrane immediately proximal to the opercular base-line 

 shows a special accumulation of nuclei (c. s.), towards which a 

 number of muscle-fibres radiate through the body- cavity from 

 both lateral walls. When the calcification has advanced so 

 far as to mark out the future orifice, two lateral calcified 

 processes and a median tongue-like structure begin to grow 

 up just proximal to the operculum (fig. 14, I. p., tg.). The 

 nuclei are arranged in a more definite mass along the proximal 

 margin of the orifice, to which the lumen of the tentacle- 

 sheath {t. s.) now extends. Still later, the two calcareous 

 processes meet, although the suture between them is per- 

 sistent. There is thus left, between them and the calcareous 

 tongue, a crescentic pore (fig. 15), the concavity of which is 

 directed proximally. This is an aperture of the compensa- 

 tion-sac, which, however, opens to the exterior at the proximal 

 edge of the operculum as well, an arrangement which is most 

 obvious in the fertile zooecia. The occlusor muscles of the 



