34 Revieivs — Development of Monticuli'poroids. 



Cheilostomata; and of these the last three may be derived each 

 independently from the first. It is to the Cyclostomes, then, that we 

 look for the primitive and typical development. Barrois^ andHarmer^ 

 have eacli worked out in detail the development of Tuhulipora, a recent 

 Cyclostome of primitive standing. But, as far as the skeleton is 

 concerned, the earliest stages can be studied with ease in the still more 

 primitive Stomutoporas well pi'eserved as fossils in British Jurassic 

 rocks.^ On settling, the Polyzoan larva secretes a shell of spherical 

 shape — the protcecium,'* from which springs the first individual — the 

 ancestrula.^ In the most primitive forms {Stomatopora) the ancestrula 

 gives rise to one or two buds ; in the more advanced though still 

 primitive forms {Bere^iicea, Tahulipora) three buds spring from the 

 ancestrula, and these in turn give rise to a varying number. In 

 Berenicea and other discoid forms the later buds ciirl round and may 

 cover the protceciutu and often the ancestrula too; and this tendency 

 has caused difficulty in demonstrating the protoecium in certain 

 Palaeozoic forms. Generally speaking, then, the typical early stages 

 of the Cyclostome appear as an ancestrula with a bulbous proximal 

 end (the protoecium) and with one, two, or three distal buds. The 

 Cryptostomata show similar stages before taking on characters peculiar 

 to themselves.' In the Cheilostomata development is condensed by 

 tachygenesis, in accordance with their specialization, to such an extent 

 that the protoecium is merged in the ancestrula,' which itself has 

 already lost the primitive tubular form. Among the Trepostomata 

 Cumings has now demonstrated the globular protoecium and ancestrula 

 with three distal buds in Prasopora, Callopora, and Phylloporina, and 

 the post-protcEcial stages in Peronopora, Rhomhotrypa, Amplexopora, 

 and Homotrypa ; in Rhomhotrypa and Amplexopora, however, the 

 ancestrula apparently gives rise to two buds only. 



It is now possible to form some idea of the inter-relationship of the 

 four Orders. Tlie Cyclostomes undoubtedly are the most primitive, 

 since they alone show the complete demarcation between the protoecium 

 and ancestrula — the two appear as separate entities. The Trepostomes 

 show early stages exactly comparable with Cyclostomes, except that 

 the demarcation between protoecium and ancestrula is hardly to be 

 seen, the latter appearing as the distal end of the former. The 

 characters separating Trepostomes from Cyclostomes concern the 

 characteristic habit of growth, the nature of secondary skeleton, and 

 matters of heteromorphism ; but the tubular zooecium and simple 



' Barrois, Recherches sur Vembryologie dcs Bryozoaircs, 1877, pp. 70-85, 

 pis. iii and iv. 



■ Harmer, "On the Development of Tubtilipora" : Quart, Journ. Micros. 

 Sci., vol. xli, pp. 73-157, 1898. 



■* Lang, " The Jurassic forms of the 'genera' Stomatopora and Proboscina^^ : 

 Geol. Mag., Dec. V, Vol. 1, pp. 315-22, 1904. 



"* Cumings, " Development of some Palaeozoic Bryozoa " : Amer. Journ. Sci., 

 ser. IV, vol. xvii, p. 50, 1904. 



^ Jullien, Mission Scientifiqiie du Cap Horn, 1882-3, torn, vi, Zoologie — 

 Bryozoaires, p. 29, 1888. 



•^ Cumings, op. cit., pp. 58-74, 1904. 



'' Cumings, "Development of Fenestella^^ : Amer. Journ. Sci., ser. rv, 

 vol. XX, p. 170, 1905. 



