THE MORPHOLOGY OF THE OHEILOSTOMATA . 317 



Steganoporella alveolata^ Harraer.^ 



I have previously alluded' to the muscular system of this 

 species. That of the B-zooecia is more highly developed 

 than in auy other Polyzoou with which I am acquainted. 



The B-opercnlum has the form shown in figs. 64, 66. Its 

 mode of articulation differs in a striking way from that of 

 the Cheilostomes previously considered. Although its base 

 is continuous with the unmodified frontal membrane which 

 stretches across the whole of the proximal half of the 

 zooecium, a strong basal sclerite {h. s.) is differentiated. Into 

 the two ends of this are inserted, beneath the frontal mem- 

 brane, two ligaments {lig.) which, continuing the direction 

 of the sclerite, become attached to the strong condyles {cond.). 

 The operculum is thus slung by two tight transverse cords 

 between the two condyles, and this arrangement constitutes 

 the hinge. The basal sclerite probably has the further 

 object of playing over the projecting part of the structure 

 which I have called the median process (fig. 60, in.. X'V-), and 

 of preventing the operculum from being pulled too far by 

 the contraction of the enormous occlusor muscles. Of these 

 there are two pairs, which originate, with the divaricator 

 and depressor muscles, deep down within the lateral recesses 

 at the sides of the median process. The insertion of the 

 occlusors is shown in fig. 64, One pair [occL], which pro- 

 bably correspond with the normal Cheilostome occlusors, 

 pass by strong tendons (tend.) into the occlusor tubercles 

 [occl. t.) of the 0[)erculum, but the tendon is also continuous 

 with a fascia (/.), which is connected with the strongly ])ro- 

 jecting proximal end of one of the main sclerites [m. s.) of 

 the operculum. The second pair, which I distinguish as the 

 distal occlusors [occl.'), are inserted into a strong transverse 

 fascia (/.') which lies between the two main sclerites. In 

 fig. 64, which represents an operculum torn away from its 

 zooecium, this fascia has been displaced from its natural 



1 1900, p. 287. 



2 Loc. cit., p. 230. 



