134 MOLLUSCA. 



CLASS POLYZOA (BRYOZOA). 



The animals of this class are minute, compound, aquatic and 

 mostly marine. They usually form moss-like or coral-like cal- 

 careous or chitinous masses, each cell containing a worm-like 

 animal. 



The digestive tract is bent so as to bi'ing the anal aperture near 

 the mouth. Retractor and adductor muscles draw the body in 

 and out of the cell. A crown of tentacles surrounds the mouth. 

 There is no heart or vascular system, but a nervous system of one 

 or two ganglia and radiating nerves. They are bisexual, and 

 multiply by budding or by eggs. The avicularia or "bird's- 

 head process " is a peculiar appendage of uncertain function. 



The derma-skeleton (coencecium or polyzoarium) of the polyzoa 

 resemble and are often mistaken for " corals " ; but the separate 

 beings or zooids of the polyzoon are connected only by the integ- 

 ment, and not through the body cavity as in the coelenterates. 



In one fresh-water type {Cristatella) the colony creeps upon a 

 flattened base. Some marine forms live at great depths. The 

 type began in the Cambrian and has persisted without great 

 change. The class probably culminated in the Cretaceous period, 

 over 200 species occurring in the chalk. 



No. 409. Archimedes laxa, Hall. 

 Sub-carboniferous, Logan Co., Ky. 



No. 410. Fenestella retiformis, Schloth. 



This is the most common genus of the Palaeozoic forms, and especially 

 characteristic of the Carboniferous. Permian, Poesneck, Thuringia. 



No. 411. Gorgonia antiqua, Hall. 

 Trenton Limestone, Holland Patent, N. Y. 



No. 412. Ptilodictya Shafferi, Meek. 



This genus is especially characteristic of the Carboniferous. Tlie coenoecium 

 is flattened, foliaceous or often dichotomously branched. Cincinnati Group, 

 Cincinnati, Ohio. 



