45 



opening through which the tentacles and oral extremity of the polypide are 

 protruded. The terms orifice, oral aperture and mouth are inaccurate and con- 

 fusing and the proposed name will I think prove ad\antageous«. 



It is evident from the above morphological considerations on the operculum, 

 that when we exclude the small number of species which are furnished with a 

 simple operculum, we have in all other Cheilostomatous Brijozoa, on the frontal 

 side of the zocecium, a larger or smaller opening (viz. an uncalcified portion) 

 which is covered by an operculum in connection with a larger or smaller portion 

 of the original frontal membrane. The relation between this portion and the 

 opercular valve may be very different both in regard to the mutual size of the 

 two portions and to their nature. In the Malacostega both are generally mem- 

 branous and the opercular valve is as a rule many times smaller than the rest 

 of the cover. We lind a comj)letely chitinized opercular valve however in a 

 number of Onijchocelhi species (PI. XXII, fig. 3 b), as well as in a number of species of 

 the family Scrupocellariidae, and in quite a number of Meinbranipora forms the 

 suplementary cover is greatly reduced in extent. This is for instance the case in 

 Callopoia luina.v, C. trifolium, certain varieties of C. Fleniincji, Kosscliaiui Rosseli 

 and Membvaniporina artjentea Mac Gill. ', in the last of which it may be smaller than 

 the opercular valve. In the Ascophora the suplementary cover, or as we before 

 have called it the accessory portion, is frequently fused together with the oper- 

 cular valve to a well chitinized, compound operculum, but in quite a number of 

 forms (e. g. in Discopora species, certain l'^sch<tn>idcs species, etc.) the structure of 

 the operculum is not different from that we find in the Malacosleija, because the 

 opercular valve as well as the suplementary cover is membranous. On the other 

 hand we find in a smaller number of Malacoslega a well-chitinized, compound 

 operculum, as in Cluiperia s])in()S(i, Cli. aipensis and Me<]upor(i rintjens, and 1 do 

 not doubt that ^LepruUct^ Poissoni and Dorij/xtrella spathiilifera', both of which 

 have a well-chitinized, compound operculum, must also be classed with the division 

 Mdlctcoslec/a. 



For these reasons we propose to keep the term > aperture «. which Johnston 

 uses, for the frontal zoreciai opening in all Cheilostonutta; for, even though it might 

 be right to use a special term for the o[)ening covered by a simple operculum, 

 two separate terms would i)e unpractical, as the forms provided with a simple 

 operculum occur as a rule in families together with forms which have a compound 

 operculum. It can always be settled, by examination of the form of the oper- 

 culum and the aperture, the position of the hinge-teeth ami ol the corresponding 



' 74. vol. T, PI. SI, fig. 2 - 84, p. lOfi. 



