vui PHYLUM MOLLUSCOIDA 341 



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 ORDER 2. PHYLACTOL^EMATA. 



Fresh-water, Ectoprocta with horse-shoe-shaped lophophore and 

 with an epistome. 



Including Cristatella, Plumatella, Fredericella. 



Sub-Class II. Endoprocta. 



Colonial or solitary Polyzoa multiplying by the formation of 

 buds, which in Loxosoma soon become separated off, while in 

 Pedicellina they remain connected together by a creeping stolon. 

 The anus, as well as the mouth, is internal to the lophophore. 

 The introvert is slightly or not at all developed. A pair of ciliated 

 nephridial tubes are present. 



Systematic Position of the Example. 



Bugula avicularia is an example of the sub-order Cheilostomata 

 of the Gymnolaemata. It is a member of the family Bicellariidse, 

 which is characterised by the erect plant-like colony, with narrow 

 compressed branches, and attached by root-like fibres ; by the 

 avicularia, when present, being stalked and bird's-head shaped ; and 

 by the wide oblique apertures of the zooecia all facing in the same 

 direction. Bugula differs from the other genera of the family in 

 the arrangement of the zooecia in double or multiple rows, in their 

 close union, and in the avicularia, when present, being on the side 

 on which the mouth is situated. The various species differ in the 

 exact shape of the zooecia and of the avicularia. 



3. GENERAL ORGANISATION. 

 Sub-Class I. Ectoprocta. 



The Ectoprocta and the Endoprocta differ so considerably from 

 one another that it is advantageous to deal with them separately. 

 The Ectoprocta are all colonial the colonies being capable, in 

 most cases, like the colonies of hydroid zoophytes, of increasing in 

 size to an apparently indefinite extent by continuous budding. 

 The thickened cuticle which forms the support of the colony is 

 sometimes gelatinous, sometimes chitinous, sometimes chitinous 

 with sand-grains affixed, sometimes calcareous. The form of the 

 colony varies in different families and genera in accordance with 

 differences in the shape of the constituent zooecia, and differences 

 in their mode of budding and consequent arrangement. The 

 zooecia are sometimes tubular, sometimes ovoid, sometimes poly- 

 hedral. In some cases the buds are so developed that the colony 

 assumes the form of a thin, flat expansion, which may be encrusting, 

 and consist of a single layer of zooecia in close contact with one 

 another or connected together by tubular processes ; or. may be 

 erect, and with the zooecia either in one or two layers : sometimes 



