ZEA.] 



MOLLUSCOIDA. 



289 



spinosus, Owen & Shumard, 1852, (Pote- 



riocrinus spinosus,) Jour. Acad. Nat. 



Sci. Phil., vol. 2, p. 91, Kaskaskia Gr. 

 spinuliferns, Worthen, (in press,) (Pote- 



riocrinus spinuliferus,) Geo. Sur. 111., 



vol. 8, p. 90, Kaskaskia Gr. 

 stimpsoni, Lyon, 1869, Trans. Am. Phil. 



Soc, vol. 13, p. 465, Subcarb. 



subtumidus, see Eupachycrinus subtu- 

 midus. 



troostanus, Meek & Worthen, 1860, Pme. 

 Acad. Nat. Sci. Phil., p. 390, and 

 Geo. Sur. 111., vol. 2, p. 186, Burling- 

 ton Gr. 



wortheni, Hall, 1858, Geo. Sur. Iowa, p. 

 683, Kaskaskia Gr. 



•o+o* 



SUBKINGDOM MOLLUSCOIDA. 



CLASS BRYOZOA. 



The Bryozoa are small animals that grow in clusters, forming branched or moss- 

 like compound structures. Each animal lives in a separate cell, called a zocecium, 

 into which' it can retract itself, though some connection exists between the animals. 

 •The Flustra or Sea-mats, abundant on the shores of the ocean, and the moss-like en- 

 crustations so common on marine shells, are examples. All known Palaeozoic Bry- 

 ozoa were marine, and lived in calcareous cells, forming a mass that is often difficult 

 to distinguish from the true corals. 



This calcareous mass or skeleton is called the bryozoum or zoarium. It is found 

 encrusting other objects, or standing on a foot-stalk, with basal attachment, and, in 

 other cases, apparently free. There are rarely any such calcareous partitions in the 

 cell-tubes as abound in the true corals, and the method of reproduction was exclu- 

 sively gemmiparous, while the true corals were increased by both gemmiparous and 

 fissiparous reproduction. 



The animal consists of a bent tube or alimentary canal, having an oesophagus, 

 stomach, and intestine. The two orifices of the canal are situated close together, 

 but the anal opening is beyond the ring of ciliated tentacles that surround the 

 mouth. Thus constituted, the alimentary canal is inclosed in a sac having two 

 openings corresponding to the two extremities of the canal. Generally the upper 

 side of this sac is flexible, and admits of being invaginated, so that when the ani- 

 mal retracts itself into its cell the inverted portion forms a sheath around the ten- 

 tacles. Ova may be developed in a receptacle attached to the zocecium, called the 

 ooecium, or in an inflation of the surface of the zoarium, called a gonocyst. The 

 gonoecium is a modified zocecium. The term ocecia is also applied to these struc- 

 tures. Many Bryozoa have appendicular organs called avicularia and vibracula. 

 The avicularia may be pedunculate, and sway to and fro, or fixed and firmly at- 

 tached to the zocecium. The vibracula are flexible, bristle-like structures, set in the 

 excavated summit of a knob-like elevation or blunt spine. 



Some naturalists refer the Monticuliporidae and Stelliporidae to the Bryozoa, 

 and probably the latter should be so classed on as good grounds as the Fistu- 

 liporidae are referred to the Bryozoa. The Palaeozoic Bryozoa are referred to an 

 order called the Gymnolaemata, which are supposed to have had a complete ring of 



