41 



been proved for some of these forms Ihal the compensation-sac opens out through 

 this pore and there is hardly any leason lo doul)l, that this applies to all. Most 

 of them have a sini])le operculum, and a compound operculum is only found 

 in species of tlie genera Onchoporclla and CalweUia (PI. XIII). 



Just as a sim{)le operculum may either be quite chitinized or only separated 

 from the covering membrane by a basal sclerile, a compound operculum may also 

 be formed in these two ways. While for instance we have a completely chitinized 

 operculum in the genera Schizoporella, Eschariiui, Microporella, Myriozonni as also 

 in certain Cluiperia species, the compound operculum in other forms is membra- 

 nous or very slightly chitinized and only separated from the basal wall of the 

 compensation-sac by a basal sclerite. This is the case for example with the oper- 

 culum in Enriislnnielld fornminiriera, Hippopodina fegeensis, Cheihpora sincera, 

 Smittina porifera, in numerous species of the genus Holoporella and in all mem- 

 bers of the family Onchoporidae. In most species of the genera Escharella and 

 Ksclmroides, in all species of the genus Discopora and in a number of species 

 within the genera Smittina, Holoporella and Petralia the aperture is covered up 

 by a membranous operculum, the distal part of which is formed by an oper- 

 cular valve, while the proximal portion goes immediately over into the compen- 

 sation-sac. 



The connection between the operculum and the zooecial aperture in the 

 Cheilostomata may take place in two different ways, which in a few instances 

 are used at the same time, namely partly by direct connection between the 

 proximal edges of the aperture and of the operculum, and partly by a connec- 

 tion of very slight extent which occurs at each end of the hinge-line. The first 

 mode of connection is found in all the Anaska. and in those Ascophora in which 

 the compensation-sac oj)ens out through a median pore separated from the aper- 

 ture of the zooecium. The simultaneous occurrence of both modes of connection 

 is found in a number of acanthostegous and coilostegous forms, for instance in 

 Fi<inlina fupdaris, as well as in the genera Tlialamoporella and Stenanoporella (PI. V), 

 and in the last mentioned genus, the species of which have an usually large 

 and, owing to its chitinous armature, heavy operculum, this last connection is 

 very firm, the tip of the strong and conical hinge-teeth being connected by a 

 rather long and cylindrical chitinous band with the proximal corners of the 

 strong opercular arch. 



In all Ascophora, the compensation-sac of which opens out through a part of 

 the aperture which can be closed by the proximal or accessory portion of the 

 operculum, the operculum is only in firm connection with the aperture on each 

 side of the hinge-line, and in most forms which have a well-developed opercular 



