46 



parts of the operculum, which part of the aperture corresponds to the opercular 

 valve, and if we require a special expression for this we may call this »the 

 valvular aperture-. 



Polymorphism in the Bryozoa. 



As is known polymorphism also occurs in the ISriiozoa, but in contrast to 

 the case in the Hydroid polyps it is not present in all species, even not in all 

 genera or families. We can distinguish between four main forms of individuals 

 (Bryozooids): 



Aiito:oa'cia (Auiozooids), which contain a polypidc, consisting of a tentacular 

 apparatus and a well-developed digestive canal. 



Heterozowcia (Heterozooids), which have no intestinal canal, and at most have 

 a trace of a polypide in a small cell-body, furnished with a circle of fine bristles. 

 The chamber contains a strong muscular apparatus for moving the operculum, 

 which sometimes only covers the aperture, in which case the Heterozooecium is 

 called an Avicularium, and sometimes extends beyond this in the form of a whip, 

 as in the so called vibraculum, but otherwise there is no sharp limit between 

 these two forms of heterozooecia. 



Kennzoa'cia (Kenozooids), which not only have no polypide, but as a rule no 

 aperture and always no operculum. While the Autozoo^cia might be regarded as 

 alimentary individuals, the Heterozooecia as defence individuals, the Kenozoojcia 

 must be regarded as supporting, fastening and connecting individuals. To this 

 class of individuals belong: the segments which compose the thread-like basal 

 parts in numerous CJenostomala, in the Cyclostome genus Crisia, and a smaller 

 number of Cheilosionmtn (Hiiskia, Alijsidium etc.), the segments which form the upright 

 stems in Slirparia, Ahjsidiiim and CIdidonia, and certain portions of the branches 

 of the last, the radical fibres and the chambers for the insertion of the radical 

 fibres in Scnipocellariidae, Caleiuiriidae etc., the peculiar lateral compartments 

 in the Catenariidae, the modified marginal individuals, which appear for in- 

 stance in Flustra securifrons and Fl. carbasea, the small chambers which form 

 the encrusting base and the outer (ba.sal) layer of the Reteporn colonies, the 

 peculiar cavities which appear among the zocecia in Membranipnra Lucroi.vi, 

 Cribrilina Udimnryinata etc., the supporting tubes in the Cijclostonudd, as also the 

 small chambers which surround the oa-cia in the family llippollwldae, in nuuiy 

 species of the family Cateiutriitlae etc. (see under ooecia). 



As a fourth class of colonial individuals we may perhaps in many species 

 regard the egg-producing individuals (Gonozmn-ia). While in numy cases, e. g. 

 in Meiubniniponi meinbraiiuceu, all individuals in the colony seem able lo produce 



