284 



MOLLUSCOIDEA BRYOZOA 



SUB-KINGDOM V 



Sub-Order D. CHILOSTOMATA. Busk. 



(Bryozoaires cellulines, d'Orbigny.) 



Zooecia oval, turbinate, urceolate, quadrate, or hexagonal, arranged usually side by 

 side. Orifice more or less anterior, of smaller diameter than the zocecium, closed by a 

 movable cover. Ova commonly matured in external marsupia. Appendicular organs 

 d. 



True Chilostomata are first met with in the Jura, but their progenitors are undoubtedly to 

 be looked for in the Palaeozoic Cryptostomata. They attain an astonishing development in the 

 Upper Cretaceous, and in the Tertiaries and existing seas they greatly surpass the Cyclostomata 

 in number and variety of species. 



Not all of the Chilostomata have a completely calcified zoarium, some being corneous and 

 flexible (Flustridae), and others having the front wall of the zocecia more or less membraneous 

 and the rest calcareous (Membraniporidae). Consequently, in fossil examples of the latter, the 

 zooecia are entirely open on the upper or front side (Fig. 477). Avicularia and vibracula are 

 very commonly present, and are indicated in fossils by the " special pores " in which they were 

 lodged. External ovicells are more commonly developed than in the Cyclostomata, and usually 

 occur as rounded, blister -like cavities in front of the zooecial apertures. Reproduction by 

 gemmation takes place at the growing edge of the colony, the young cells arising from the 

 anterior end or from either side of the parent cell ; and repeated gemmation almost always 

 results in a more or less regular arrangement in series. Direct communication between adjoining 

 zooecia is effected by means of small perforated plates (communication plates, Roscttenplatteri), 

 set in corresponding positions in the side walls of each zooecium. 



The classification of the Chilostomata is as yet in an unsettled condition. The older systems 

 of d'Orbigny and Busk were highly artificial, undue prominence having been given to zoarial 

 modifications ; but through the labours of Smitt, Hincks, and Waters, who have demonstrated 



the much greater importance of 

 zocecial characters, a decided advance 

 lias been made. Only brief descriptions 

 of the more important genera, or those 

 having numerous fossil representatives, 

 can be introduced here. 



Family 1. Eucrateidae. Busk. 



Zoaria branching, erect, and free, 

 or recumbent. Zocecia uniserial or 

 biserial, pyriform, with a sub-terminal 

 and usually oblique aperture. Avicu- 

 larian and vibracular appendages 

 wanting. Cretaceous to Recent. 



Eucratea, Lamx. (Fig. 471). Zoaria entirely decumbent, or composed of a 

 creeping, adherent base and erect branching shoots. Zooecia calcareous or sub- 

 calcareous, rising one from another so as to form single series. Branches springing 

 from the front of a zocecium below the aperture. 



FIG. 471. 



Eucratea, labiata, Novak sp. Cenomanian ; Velim, Bohemia. 

 A, Zoarium, Vi- -B> Zooecia, three of them with fractured walls, 

 highly magnified (after Novak). 



Family 2. Cellulariidae. Busk. 



Zoarium erect, usually jointed, dichotomously branching, phytoid. Zooecia in two or 

 more series, closely united and arranged in the same plane. Sessile avicularia and 

 vibracula generally present. Tertiary and Recent. 



Cellularia, Pallas ; Menipea, Caberea, Lamx. ; Scrupocellaria, Van Benecl. Miocene 

 to Recent. 





