vin PHYLUM MOLLUSCOIDA 321 



avicularia, when present, being stalked and birds-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 

 the lamellar colony thus formed may be fenestrated or divided into 

 lobes ; sometimes it is twisted into a spiral. In other cases the 

 colony, instead of being lamellar, has the form of an erect, shrub- 

 like structure, consisting of numerous cylindrical, many-sided, or 

 strap-shaped branches arising from a common root. Sometimes 

 there is a creeping cylindrical stolon, simple or branched, having 

 the zooids arranged along it in a single or double row. The colony 

 is free only in Cristatella (Fig. 258, bis) in which it performs 

 creeping movements, and in one family of the Cheilostomata the 

 Selenariidce in which it moves along with 'the aid of certain 

 peculiar appendages the vibracula to be described subsequently. 



The zooecia open on the exterior by means of circular, or semi- 

 circular, or crescentic, apertures, which in the Phylactolaemata and 

 the Cyclostomata among the Gymnolsemata are devoid of any special 

 closing apparatus, while in the Cheilostomata there is a movable 

 lid or operculum closed by a pair of occlusor muscles when the 

 introvert is retracted, and in the Ctenostomata there is a series of 

 lobes or teeth which close in together over the opening. The 



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