35 



used by several writers, is therefore not quite correct, and it would be far more 

 correct to compare it with a division in a modern purse, in which the latefal 

 walls when the purse is closed are folded into this. To explain the formation of 

 such a vestibulum a simple horizontal dividing of the flat vestibular rudiment is 

 not sufficient, as this would only lead to the formation of the zocecial and the 

 opercular walls. To explain the formation of the two free lateral walls it must, 

 I think, be necessary to suppose that an invagination on each side has taken 

 place together with the division of the choi-d-shaped rudiment. 



The distal part of the vestibulum presents a number of differences, partly in the 

 way in which it is fastened to the operculum, partly in its structure and nature, 

 arid we may here shortly mention some of the differences, the closer study of 

 which however will require fresh investigation-material. While its frontal wall 

 in a number of forms is fixed directly to the free edge of the operculum, as in 

 most Ffustra species, Membranipora membranacea, Electra pilosa, Gemellaria loricata, 

 Microporina borealis, Scuticella plagiostoma, Retepora Beaniana, etc., in a number of 

 other species it is fixed at a shorter or longer distance within the edge, in such 

 a way that we must conclude that the frontal and basal walls have moved from 

 each other after the division of the vestibular rudiment. For instance we find 

 this the case in Flustra abyssicola, 11. carbasea, in numerous members of the 

 family Membraniporidae (^Callopora aurita, Tegella unicornis, Memb. arctica etc.), in 

 the family Scrupocellariidae, in the genera Steganoporella, Bicellaria and Discopora, 

 in *Lepraliai^ Pallasiana, Tubucellaria opuntioides etc. In most of the Cheilostomata, 

 and as it seems in all Anaska as well as in numerous Ascophora, the part of the 

 frontal wall of the vestibulum, which adjoins the operculum, is more or less 

 chitinized, whether this connection takes place in or within the edge of the oper- 

 culum, and when such an operculum is isolated the chitinized portion of the 

 vestibulum adheres to it as an arched chitinous ridge (the »opercular arch«) rising 

 from its inner surface, which is lowest at its distal, central part, but which gener- 

 rally on each side ends in a more or less triangular » flange «. which is a part of 

 the above-mentioned lateral wall of the vestibulum and which goes directly 

 over into the membranous part of this. If we compare opercula of the youngest 

 and the oldest zocecia in a colony, we sometimes (e. g. in Microporina borealis) find 

 that the opercular arch is higher on the latter, and now and then this opercular 

 arch sh&ws distinct lines of growth. There is a cavity between the operculum and 

 the frontal part of the vestibulum, the opercular cavity, and into this extend the 

 occlusor muscles of the operculum, which in the forms with a well-developed 

 opercuiai- arch are generally fastened to this; if not, the opercular muscles are 



attached to the inner surface of the operculum itself. 



3* 



