POLYZOA, BRACHIOPODA 537 



Order Gymnolaemata. Ectoprocta mostly marine, with a circular 

 lophophore, without an epistome. 

 Suborder Cyclostoniata with tubular zooecia, aperture without 



operculum, embryonic fission characteristic. Crisia. 

 Suborder Cheilostomata, with aperture of zooecium closed by an 



operculum. Bugula, Flustra, Membranipora. 

 Suborder Ctenostomata with aperture of zooecium closed by a 

 folded membrane when the lophophore is retracted. 

 Alcyonidium, 



It is, however, possible that the Endoprocta should be separated 

 from the Ectoprocta as a distinct phylum. 



PHYLUM BRACHIOPODA 



Coelomate unsegmented animals with a bivalve shell which is always 

 attached, the valves being respectively dorsal and ventral in position; 

 a complex ciliated circumoral organ, the lophophore, which maintains 

 a circulation of water in the mantle cavity and leads food currents to 

 the mouth. 



The group contains only marine animals with a strong but super- 

 ficial resemblance to the lamellibranchs among the Mollusca. In the 

 Palaeozoic and Mesozoic it was more abundantly represented than 

 the Mollusca, but at the present day it contains but few genera and 

 species. Of the former Terehratula and Waldheimia are found in deep 

 water off our own coasts and Crania occurs abundantly in shallow 

 water in the West of Ireland. Lingula is not found in Britain, but in 

 the tropics is sometimes exceedingly abundant in mud between 

 tidemarks. ^ 



In such forms as Waldheimia and Terehratula (Figs. 397, 398), the 

 ventral shell valve is larger than the dorsal and has a posterior beak 

 or umbo perforated by a round aperture through which passes the 

 stalk for attachment to a stone or rock. Each valve is secreted by a 

 corresponding mantle flap, but in a way which differs from the corre- 

 sponding process in the Mollusca. The mantle epithelium is produced 

 into minute papillae which traverse the substance of the shell. The 

 cells, of which the papillae are composed, are often of a minutely 

 branching type which resemble the bone corpuscles of vertebrates. 

 It must be supposed that the papillae are concerned with the secretion 

 and growth of the shell. Each valve (Fig. 399) is composed of an outer 

 layer of organic material (periostracum), under which is a thin layer 

 of pure calcium carbonate and a thick inner prismatic layer composed 

 mainly of calcareous but partly of organic material. The shell valves 

 are opened and closed by a muscle system which is much more 

 complicated than that of the lamellibranchs. ^'*''C\C A I ^ 



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