THP^ MORPHOLOGY OF THE CHEILOSTOIFATA . 305 



The principal interest of this species, in the present connexion, 

 is as an illustration of the group of Cheilostomes in which 

 there is but a single series of conspicuous marginal pores or 

 areolee (ar.). 



Catenaria lafoutii,^ And. (fig. 49). 



I consider this species in this place on account of its 

 possession of a typical compensation-sac ; but I shall discuss 

 its affinities later. The zooecia are very long, curved_, narrow 

 proximally, and dilating distally. From the back of the 

 distal end usually originate a pair of new zooecia (z.'), each 

 starting in a calcareous base containing a body-cavity, into 

 which is fitted a chitinous joint [j.), which forms the connexion 

 with the younger zooecium. The oral extremity of the 

 zooecium is surrounded by a circlet of short, hollow spines 

 {sp.). The ovicell (or.) is placed obliquely on the distal 

 side of the orifice, and an extremely strong suboral avicu- 

 larium {avic.) guards the entrance to the compensation-sac. 

 There is some evidence that the base of the avicularium 

 has been derived from a single pair of suboral frontal 

 spines. The porous region of the zooecial wall in the main 

 cor)'esponds with the limits of the compensation-sac {cs.), but 

 a few pores occur outside that region, even on the basal 

 surface. 



The compensation-sac is extremely distinct, and is indicated 

 in a figure by Calvet (1900, pi. viii, fig. 9), who does not, 

 however, make any reference to the structure. It is provided 

 with several groups of parietal muscles (j;. vi.). The oper- 

 culum is placed obliquely, and its base is distinctly continuous 

 with the floor of the compensation-sac, the orifice of which is 

 rather large. Each occlnsor muscle (occl.) originates from 

 the partition wall between the body-cavity of the zooecium and 

 the base of the dausfhter-zooecium of its own side. 1 have 

 not distinguished any divaricator muscles. 



1 Bubk (1852), p. U. 

 VOL. 46, PART 2. NEW SEKIES. U 



