80 



of zocecial apertures, and most of these cases appear in ooecium-bearing species, 

 the ooecium-bearing zooecia having another form of the aperture from the others. 

 This condition occurs for example in most species of the family Catenariidae, the 

 species of the genus Hippothoa, in Cribrilina clithridiata Waters, ^SchizoporeUa< 

 filocincta Rss., ^Sch.- subimmersa Mac Gill, Sch. spongites Pall, ^Lepraliai- bistata 

 Waters, etc. We also find two different forms of zocecial aperture in a smaller 

 number of species which have no ooecia, for instance in a number of Stegano- 

 porella species, in Euthgris obtecta, Eiith. clathrata and ^Lepralia-^ depressa. Apart 

 from these cases, the form of the aperture is constant within the same colony, 

 and in the main constant within the species, though now and then it may show 

 distinct variations in colonies from different places. This last condition is found 

 for example in Schizoporella spongites (PI. XVIII, fig. 4 c, d). On the other hand, 

 the form of the aperture may have a rather diff"erent character in species, be- 

 longing to the same natural genus, and we may mention here some examples. 

 In species of the genus Thalamoporella (Pis. VI, VI a, VI b, VI c) we frequently find a 

 more or less sharply marked, wider or narrower, rounded sinus, but it is at 

 times so faintly marked, that the aperture becomes irregularly circular and in a 

 few species it is provided with a straight or almost straight proximal edge: 

 Within the genus Haswellia an oral sinus is lacking in H. gracilis (PI. XVI, fig. 1 b), 

 while it is distinctly developed in the other species (PI. XVI, fig. 2 b), and there 

 is a sinus in Exochella tricuspis (PI. XVII, fig. 9 b), which is wanting in E, longi- 

 rostris (PI. XVII, fig. 6 b). The form of the aperture also undergoes a somewhat 

 considerable variation within the genus Smittina, as it sometimes has a more or 

 less distinctly rounded sinus, sometimes is irregularly circular or quadrangularly 

 rounded. 



While the examples mentioned, which might easily be added to, make it al- 

 ready very doubtful, if it is right to attach the great systematic importance to 

 the form of the aperture, which Smitt., Hincks and other authors do, this doubt 

 is further strengthened when we examine the whole extent of the differences, 

 which the form of the aperture can present within the Cheilostome Bryozoa, and 

 the appearence of these different forms of the aperture within a series of natural 

 families. 



We can refer the numerous forms of aperture to two different types, which 

 however in reality grade evenly into one another, and which we may call the 

 »holostome« and the »schizostome«. The holostome aperture may have the form of 

 one continuous line of different shape (circular, oval, transversely oval), in which 

 case the boundary between the distal (the anter) and the proximal (the poster) 

 part of the edge of the aperture cannot be defined by the aid of the form of the 



